The cost to operate such a service, if there even is one, is likely completely negligible. Not to mention they already offer what essentially amounts to the same thing with their PTR, no charge involved. Not to mention you already pay a monthly premium just for access to their servers, so the expectation for such services to be included is completely reasonable.
Data storage is very cheap, and can be made cheaper through archival practices for inactive accounts. And players can make new characters on the TBC realm while keeping their old ones on classic, so it's unlikely this charge significantly reduces the number of characters being stored anyways.
Even if it did, charging money as a limiting factor is still poorly justified given they could easily put other limitations in places.
This really just boils down to padding executive bonuses and pleasing share holders. But if you're satisfied being hoisted with exuberant prices and extra charges for such a reason, then there's not much more I can say.
It's good that they reduced it, 15 dollars is at least a lot more reasonable. But the fact that they were able to normalizes this practice in the first place is pretty wild.