Do programmers realize that open source makes their careers less rewarding?

I don't really agree. A lot open source exists because a developer wanted to work on a project and found it rewarding, though they are paid to work on something else.

The idea is that you contribute effort in one area and get a return value in lots of areas. Some people only take and never give, but overall I'd say most developers gain a huge benefit from OS code.

I agree. In OSS philosophy, the only way to make money and stay "good" is to work on some company or through ads or providing other non-programming services. So it discourages indie/shareware model of making money through programming - the thing you actually enjoy.

The top answer on that page (by some facebook employee) confirms this.

I wouldn't find programming (as a career) to be rewarding at all if I didn't code open source software. I'd rather be paying someone else to do it. Success is too good not to share with your peers, since nobody else really understands.