Yeah, what Lelygax said.
It's systems that have a central network (PSN, XBLA) that you have a possibility of getting banned and losing your account.
Since there's no such thing as that for Wii, as long as you got a friendcode and a buddy, you can play online.
Some games even let you play online without that (Tatsunoko VS Capcom, Monster Hunter Tri' *I think*).
The only problem with online Wii ISO gaming is that if your ISO is modified in any way, you will get a disconnect because the two opposite data may not match, which would cause a problem (that's why when people play with Brawl+, Brawl-, or ProjectM, all members of the battle must have the exact same mod and version of mod, otherwise you'll desync and get a disconnect).