It's not about spite. It's about preserving the motivation to continue developing free software. It's nice thinking that the fruits of my labor will benefit everyone and that derivative efforts will as well.
The GPL gets an awful lot of flack from a community that owes it so much and I truly don't understand why.
Greed and selfishness play a role. Some folks like being able to take things without having to give in return. And they get frustrated when they find an awesome library that does what they want but they can't use it in their paid/closed product because it is GPLed. And there isn't an equivalent BSD/MIT licensed library. There are equivalent commercial libraries, but they cost money. So the dev winds up complaining about the GPL.
While I suspected as much, I figure it's worth asking in case there is a nobler reason that I'm simply unaware of. I don't want to presume too much.