Respecting freedom is better for business
Nevertheless, since every use of the GPL – whether motivated by concern for freedom or not – helps more users be free, it makes sense to talk about the potential monetary benefits of copyleft.
Because commercial use has to be allowed by a program's licensing terms for it to be considered free software, GPLed software does make a lot of businesses and a lot of people a lot of money. While the specifics in this area depend on empirical research, there are structural reasons to believe that the GPL is even better for this than lax permissive licenses.
Before we start, let's set expectations and state the obvious – people don't have an inalienable right to make money in every given way just because it's possible to do so. We know this, but it is still forgotten when people start talking about how policy impacts profit, including whether the government should take action to protect certain business models. We have innumerable laws regulating sale of controlled substances, banning various kinds of money lending, requiring minimum wages for workers, and so on. Many of the activities now restricted were allowed at some point in history. Even if the GPL turned out to be worse than proprietary terms for selling software, this wouldn't mean rejecting the GPL.
But it turns out that the GPL is very well-suited for commercial usage, and even better than lax permissive licenses. The GPL ensures that you will benefit from improvements others make and distribute to your code. With lax licenses, others, including your business's competitors, can fix bugs in and add features to your code, then share the result under proprietary terms so that you can't make use of them. When using lax licenses, you are essentially doing unpaid work for your competitors. With the GPL, you are engaging in a mutually beneficial form of targeted cooperation with them, enabling you to focus your time and resources on what truly differentiates your business. You are also maximizing the common interest everyone distributing that code has in making it the best it can be.
Similarly, the GPL also enables constructive and profitable cooperation with individual users. People purchasing and using your product will have ideas about how to make it better. Providing them with the source code and permission to make modifications enables them to actually share these improvements with you. Any free software license does this, but using the GPL sends a motivating signal to your users encouraging them to participate. It says you respect them, are committed to software that respects their freedom and protects them, and are not just dipping a toe in while keeping the option of pulling the rug out from under them. Users respond to this, and we at the FSF are happy to help you publicly highlight your commitment, to make sure our supporters know.
This is also why you should specifically use GPLv3, which prohibits locking down the devices shipping your software. When the device is locked down, users generally can't install or share their modified versions of the software without violating the Digital Millennium Copyright Act (or similar laws around the world) – a criminal offense. Users will not be able to test their innovations. This means no thriving community around your products.
Defending your business and your users against patent lawsuits is extraordinarily expensive, and the risk of such suits is a deterrent to user contributions. Protection against infringement claims is therefore a strong third financial reason to use the GPL – especially GPLv3, which strengthened patent guarantees over GPLv2. While software idea patents have been reduced by recent court decisions, they are still an enormous threat, especially to small companies and individual developers.
The advantages of copyleft become more evident over time. Lax licenses may seem appealing at first because they appear to be simpler; works under them can be combined more easily with works under other licenses. But since they allow proprietary versions in the future, they are in the long-term an invitation to the extreme incompatibility, complexity, and compliance costs intrinsic to proprietary software licenses. There may seem to be no chance of someone making proprietary versions of your software now, but that option will always be there, and as soon as someone exercises it, your space will become fragmented and difficult. To work effectively with other companies in that world, you'll end up needing additional legal structure, like trade associations or bilateral contracts.
Lax licensing is free (as long as you are also providing the source code) and so is better than proprietary terms both ethically and for getting some of the practical benefits above. But copyleft – in particular the GPLv3 – is the best choice in the vast majority of situations. Choosing it is not quite enough by itself. To be fully effective, people do have to believe that the GPL will be enforced when its terms aren't followed. Otherwise, the imperative to pass on the same freedoms one receives are just words on paper, and the GPL functions in practice as a lax license.
Because of this, the whole commercial sector benefits from nonprofits like the FSF with an agenda only of protecting user rights enforcing the GPL, preventing freeriding, and keeping the perceived strength of the license high. This is why, once you start making money from your GPLed software, you should start giving back to organizations like the FSF who do GPL enforcement in accordance with the Principles of Community-Oriented GPL Enforcement, which we drafted together with Software Freedom Conservancy. These principles prioritize the ethical goals of the license – including helping companies properly distribute free software – while holding legal action as a last resort. Support for enforcement work done in this way is an investment in your own success and future. But whether you donate or not, you can and should make use of the freedom and the commercial benefits the GPL provides for you and your business.