Some other commenters in this thread have pointed out that nobody is forcing open source developers to work on the contributions that they make, and that is strictly speaking true. But in the culture of software development, there does seem to be an ambient message often repeated or hinted at, that it is good to "give back" to open source by contributing and that it is virtuous to do so. I now am starting to rethink…
So, you can make your software free as in 'free speech', and obligate other companies to keep the products they construct above yours still free.
Better than to make them free as in 'free beer' where they can just consume your work and not give back to the overall movement.