So... The dude made a mistake. Feels bad and is going to stop making the same mistake, and you come down all medieval on his arse calling him a dumbass and "not bright." Nice... Have you ever noticed what this makes you look like? Or that in any discussion, this sort of approach will have the opposite affect of what you desire? You're clearly not "sorry". Just, to be blunt and frank a bit of an ass.
PETA has a fairly effective brainwashing technique. Most of the PETA supporters won't believe a word of this no matter how much evidence you show them. It is completely stupid and inane drivel they spout, but if you never look deeply into it (for whatever reason), it is quite possible you'll never see it. The GGP might simply have taken them at their word, "we help animals." Sure he should have checked, but not everyone has time or is quite as cynical as you might be.
As for myself, I only support the People Eating Tasty Animals variety of PETA. But I have made donations I have deeply regretted when I found out more. Never anything substantial, but I $2 here or there can add up to a nice steak, and I like my steak.