For sure, this guy called Zynga evil and said he didn't want to work for them, and that's the last thing you want showing up in Mark Pincus's news feed when he's just bought your company. But to shoot back in public and call him a parasite and the weakest person on the team? That's just vengeful and childish.
Forget censorship, it's just about having a shred of professional respect. Maybe he has a fat mother, or erectile dysfunction, or shitty dress sense. If you do something to upset me, what would you want me to post about you on Twitter? There's a line that even DHH-style "I'm funny because I'm callous" can't cross.
Any business relationship carries with it a certain amount of trust, and between a leader and their team most of all. Pierce said he didn't like Zynga (his personal opinion) and he was attacked by someone who had the ability to expose private information about his job performance, and who should have owed him better than that.
Personally, I wouldn't care if Porter said "Shay Pierce is wrong and doesn't know what he's talking about", or even if he said "Shay fucking Pierce is fucking wrong fucking" while hanging out of a porn-funded blimp. It's the intention, not the words, that I find offensive.