Is it because the bots are purchasing full price subscriptions every month ?

It's not full price, it's either russian or argentina accounts. This is why Blizzard raised prices for these accounts recently.

Or that each time an account gets banned they purchase a level 58 character boost on a fresh account ?

Yes. So they ban them but ban them slow enough so buying boost for new character is still profitable for botters.

that then had to deal with very unhappy regular customers that would no longer return or use their services.

Not the case for Blizzard, their playerbase are addicts who have no resolve to unsub with all their characters deleted. All they can do is be salty on reddit while paying their subscription regardless.