Tethering has been against the rules since day one.This, and if you do root/flash custom roms or use tethering, please, just please use it for emergencies, don't tether your PS3 or your XBOX, using 20+GB's of data, because that's what really hurts all of us.
I agree, this is not really about the rooting, it's more or less about the tethering.
Don't exceed your data quota and odds are they won't say anything. They don't bother Sprint users for it, so long as they aren't going cray, so I doubt they will bug us. I suspect all of the carriers know not to press the issue on tethering because if they end up in court over it, they will lose.
As for rooting and roms, it has absolutely zero effect on them (except when people return them for stupid reasons, which is the real reason for that rule). If you think rooting being against the rules will make me stop doing it, you obviously haven't seen my phone.