I think that's because the software comes from a similar place. You have to fight for your freedom and it takes effort, and the people that put that effort in like to feel good about it by sharing (or showing off). It's like gym-goers who like to show their hard-earned progress.
And then there's the fundamental differences in core philosophy, where a lot of friction between open and closed source projects comes in. It's warranted, but I get why it's annoying.
Them becoming raging zealots is kind of the only realistic way to defend the GPL though. If they don't, it's just going to get treated like toilet paper. I'd much rather have the angry hate mob than to be disrespected by big companies who can otherwise just get away with whatever they want.
Getting everyone to agree on a single standard.