Well, there are many positive reasons to choose the GPL and that’s up to you. But some company doing their own work and not contributing it back is not really taking anything away from you. You’d have gotten the same out of it if they didn’t use it at all.
Different folks have different priorities. For me, my top priority is for my code to be used… so I pick the most unrestrictive license I can while still protecting my own interests. For others, the code actually being used is second or third place and that’s fine.
It’s only the “forcing them to give back their changes” logic that I disagree with the way stated. a) the choice is between them using your code or not using your code… so you get the same result either way. You gain nothing. b) it immediately prohibits a whole bunch of other potential contributions for people that can’t use GPL code for one reason or another. So in the non-GPL case, you cast a wider net for contributions. In the GPL case, you get a much much smaller audience and therefore the potential contribution pool is much smaller.
It didn’t “cost” you anything. You’d have had to do that work if they hadn’t used your engine at all. One could argue that they got something for nothing… but they’d have also gotten something for nothing if they didn’t make any changes at all or didn’t distribute it outside of their own organization. (GPL only requires that you provide source to the folks you deliver the product to.)
Edit: and again, I totally respect your right to choose GPL for all of the variety of reasons you mention… but that one logic flaw always bugs me.