I'm not sure that would hold water - some (quite a lot) of phones support tethering out of the box but that doesn't mean you can go ahead and use it if your plan forbids it. There was a case a while ago in Canada where a man used his phone for tethering because "he didn't realise he couldn't" and ended up with a bill for $85,000It was later reduced to $3,243 as a "goodwill gesture"
He was routing, not using a proxy. (And it was Bell, not Rogers.)