I was initially also a bit surprised by the amount of it, but after thinking about it for a bit, Blizzard seems to have engineered the perfect conditions for this to happen:

1. Create a massively successful game for PC, which also creates a giant group of people who care about the game.
2. Make a bunch of bad decisions, which are particularly bad for the existing user base.
3. Only then release the game on Steam, meaning there are no positive reviews from the launch period and a bunch of disgruntled former players, all of which are allowed to review the game because the game has become F2P.