Because exploiting people is wrong.
I am a SWE engineer with photography as a hobby.
If a tv show contacted me to do a wedding shoot, I'd be excited for the coverage and I wouldn't feel exploited since doing shoots is fun for me.
I could see how there are parallels to the "background actor" industry where, by law, you have to pay all of the talent on screen, even if its a nominal amount and the actor would of worked for free.
But that doesn't the case here?
If a TV show contacted you do create a web app, would you do that work for free? Would you give up other opportunities to get paid to do so?
So, say no? (I've done that a couple of times in similar situations.)
No, because it is not fulfilling.
I do use my career skills to teach under privileged students how to program for free.
I too have a photography hobby (or did, pre-kids). I release everything I do that's any good via Creative Commons attribution-only licenses. My pics have shown up on articles in USA Today, Forbes, Yahoo Finance, and many many other places. Plus they are in a number of free and paid wallpaper apps. People sell prints of them on Amazon. I can go buy a cup with one of my photos on it right now. I was even interviewed for a small Discovery Channel video.
I think it's awesome. As long as they stick my name on there and follow the terms of the license, more power to them.
I've had people (especially in the local photographer's club) tell me that I'm being horrible and helping to kill the professional photography industry etc. But in my view there is just nothing wrong with giving away your creative output in exchange for credit.
Do you think people get paid when they give a radio interview? They are using the radio as a platform for selling.