tl;dr:

"Don't do this. It works, but I don't like it."

It seems like a perfectly cromulent business practice to me, unless they start suing people who didn't give them credit cards.

You use the service. You're told, after awhile, that you've racked up a bill. You keep using the service. You're told your racked up bill is bigger.

And yet, the reason you're using the service after the first bill is because you find it valuable.

You have two choices. Pay up to keep using it, or stop.

The fact that you decided to pay up to keep using it is actually, imo, a pretty good advertisement for the service.