I thought that here in the US only applied to a specific list of disallowed reasons (race/gender/etc). Which don't include "being targeted by the Internet Lynch Mob".

It depends on your contract. If it guarantees employment and you're not an at-will employee, the contract will state reasons for why your employment can be terminated. It's possible that the contract might include "being targeted by the Internet Lynch Mob" as one of the reasons, but I doubt it.

People actually use those (employment contracts) here?

I so far worked as an at-will employee, but some of the positions I'm interviewing for are explicitly 6 month or 12 month contracts.

Maybe so. I think this could be made more confusing if he was at the conference while on the company dime.