Look, Blizzard can and will cancel your account for no reason if they so choose because they put it in their agreement. Whether or not it's going to hold up in court (if anyone's even willing to take it that far lol?) is another story; but it'd be almost impossible to cancel an account for no reason at all because a human being would have to make the decision, and inherently, would have a reason even if it's "because I wanted to." So, really, everyone's arguing about nothing because...

1) Blizzard doesn't have a special team combing through millions of accounts trying to connect the dots between who's allowed to have more than 3 BNet accounts and who isn't.

2) Even if this fairly recent addition to the EULA/TOS is meant to be interpreted the way that it's currently being read, it only affects < 1% of all multiboxers in the history of multiboxing video games (can't prove me wrong on that - SDWLogic) -- Most of whom we all know. If you want to cause someone problems after March 18th, 2014, then report them and see if Blizzard will uphold and interpret the agreement in the same manner that's being discussed here.

I can't even fathom how they'd decide which accounts you're allowed to use and which you aren't, but the only things to do would be to either wait it out and see what happens or try to get a blue response through their customer support forum.