The whole "leasing it from Blizz" idea comes from the fact that the game is impossible to play without getting Blizzard's permission each and every time. Even if you have an account, you can't play without logging in through their servers, even if you then choose to play without experiencing anything online.

So, if they ban you, you can't even enjoy the game itself without their online component, because the game is impossible to play without it. They are, in short, revoking your purchase.

Not that this is a surprise. Giving the rise of online authentication and game services, I've been expecting this for a while. One of the reasons I hate Steam is because it has the potential for the same sort of thing. If the service ever decides to screw you, there is nothing you can do about it, since there is no access to the game content outside the service.