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 aweso

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.