My guess is that they evaluated how much money they would gain from doing this versus how much they would lose in subscriptions where people didn't feel like th

My guess is that they evaluated how much money they would gain from doing this versus how much they would lose in subscriptions where people didn't feel like they could adequately compete against wealthier players. Additionally, Blizzard makes their money from the longevity of their players. Someone who paid someone else to power level their characters are going to have a smaller emotional tie to their character, and are going to be less likely to continue to subscribe in the long-term (as a large amount of content has already been skipped for them).