I'm glad I don't live in the US. I'm in Canada, and on Rogers, (as far as I know) I can tether my phone all I want (mind you I only have 1GB of data/month....)
I agree, that, it should be illegal. Unless they have physical proof that you are tethering, they shouldn't be allowed to charge you any more for the services you are paying for. With that being said, they can't prove that you are tethering at all, considering you could be away from your home for an extended period of time, and you could be downloading a new app, or browsing the web, or watching videos from, say, YouTube a lot more than usual, which would mean increased bandwidth usage. Is this saying they are going to assume you are tethering because you're using more data than usual?
With all of that being said, it's kinda pathetic the US government, nor the FCC are doing anything about it.