Well anti-trust law are mostly applied on case by case basis. I believe the un-official term for judging is "workable competition". I don't know whether the ISP market passes that definition.
There are also various prohibitions about collusion. And that's a major problem with ISPs, i.e. even if there are several in the area they agree not to compete and hold prices high for example. I never saw anyone applying these laws to ISPs so far. Not sure why that is, may be government is too much in their pocket to actually apply existing law.
