People make money off of their changes initially (because they are paid to make them, or because using the software makes them more productive), and then give the source of them to the community. That’s not counter-capitalist, that’s just sharing. No reason to charge a license (and have to keep track of who’s using it, etc) when licensing the software isn’t your business focus.

Other people do the programming as a hobby, and work regular capitalist day jobs.

People do share their changes in accordance to the GPL license of projects that shared with them first. That’s not counter-capitalist, though. IBM and Microsoft have had cross-license plans. AMD and Transmeta have, too. Think of the GPL as a cross-license with a huge group. If it’s mutually beneficial, that’s consideration enough for some transactions even though there’s no cash up front. It’s a foolish capitalist who can’t see that things making or saving money in the future are as valuable as things making money now (unless the company fails for temporary cash flow issues in the meantime).

Picking up a low-cost or no-cost resource and using it to keep costs lower than a competitor is very capitalist indeed. If you own land with oil on it, why buy oil? If you own a building, why rent (other than that you could sell, lease back, and reinvest the sales price, for example)? If you can use Linux instead of SCO or HP/UX, that can mean lower costs for you. If you can use Apache instead of IIS or the GIMP instead of Photoshop, so much the better there too.

Richard Stallman isn’t asking for a portion of every dollar you make using GPLed software. Software authors are using their very right to individual and group ownership of property to keep GPLed software from being used counter to their desires.

Sure, certain people in the software world think all property rights are bunk, but there are leftists everywhere. Many people use public collaboration because it’s much more efficient than reinventing wheels everywhere. Modern Capitalism (big c) is largely about maximizing return on labor through efficiency.