Assuming banning bots cancelled a paying sub then the equation from their end is simple - Cost of banned bot accounts vs cost of accounts lost due to anger over botting. If 10% of their published sub numbers are botting accounts (and thus paying accounts in theory), then to justify banning the botting accounts (from a financial standpoint) they would need the number of accounts that quit due to rampant botting to be over 10% of their sub numbers. Otherwise inaction is better. Taking action to ban bots costs them the bot accounts plus whatever the costs of tracking and implementing the ban are.
Basically, the cost of tracking down, breaking, and then banning bots might exceed the cost of players leaving due to botting. If that is the case, then there is no reason for them to ban the bots. Everyone constantly reminds people that Blizzard is a business and so reputation and player satisfaction have to be balanced against costs.
They might not care at all about massaging numbers, but rather about the bottomline. The bottomline might simply be that it's more profitable to not ban the bots.