Blizzard did not do that. Valve (the company behind Steam) did that. Much salt was sown around the Blizzard offices during the lawsuits that followed. Valve won.
Blizzard went on to release a remake of the game that spawned DOTA which was universally reviled, a large part of which was due to them killing incentives to build custom maps with their new IP rules designed to avoid another DOTA incident.
Armchair analysis with no expertise or special access to information, accuracy of statements may vary.
