A] It's not your place to agree or disagree with it. You're not German, you don't live in Germany, so don't worry about it unless you plan on visiting there any time soon. That said, regardless of someones occupation - when they're in another country, I'd pretty much think that it's common sense to obey their laws. Calling wrestlers 'actors' is a bit of a stretch, because I'm fairly sure that any movie or play with the salute imagry in it wouldn't get too far in Germany in the first place.
B] It's common knowledge from where I sit. Anyone who's seen any sort of information piece on WWII and it's aftermath should've known about these laws. It's a total disrespect of the fans in Germany who put money in the wrestlers' pockets for that show, and I hope the piece of shit gets everything he has coming to him.
Think of it like this: Whether or not it's a 'banned act' I'm pretty fucking sure that the American fans would've been upset if some wrestler came out on TV and re-enacted the events of 9/11 with toy planes and whatnot.