It's just an angle of attack. They couldn't just say "hey, the user has no right to block ads!". But of course they don't like it to begin with.

According to the article, ABP claims that 90% of the whitelist did not pay to be on it.

@btdollar, just to note, your account is hellbanned.

I thought the strategy was not to tell them?

Generally, yes, because hell-banning is for trolls, bots, and those who routinely make bad posts. However, sometimes someone gets hell-banned for one extremely down-voted comment, but in general are making decent comments. It's a common courtesy to inform these people if you run across it.

What does it mean to be hellbanned and why can we see the posts anyway?

There's a user setting to view dead (banned) posts. If you see them then you have yours enabled. But nobody can reply to a dead post.