I'd think that most ISP's terms and conditions don't specify the amount of bandwidth used for hosting public websites. They just don't allow server use in general, so it's a grey area and it could be enforced if needed.
It does, however, have a policy against 'excessive usage', which is left conpletelt undefined and they have a policy of 'Acceptable Content' which is equally vague and ill-defined.
So I guess they do have recourse if i started hosting a pornsite and it became popular, but that's about it.
I don't particularly agree with it, but thats what it is.
If an ISP is concerned about traffic volume, take out the botnets hammering away at the doors — the occasional guy's private website getting slashdotted is small, temporary potatoes.