Guilds are shared?

Why is it that when having a couple of dif. chars on 1 account, if one account is with a guild, then the other chars also have the same guild...
Is there a way to go *around* this?

No there isn't. It keeps people from having one character joine one guild, know their tactics in the arena(s) and then during a G vs. G match, use a character in another guild and relay that information/work to undermine the other guild on it's standings.

Basically the guilds are account based, not character based.