They never had to offer money for the Mod Community to make quality maps in the past. And pay close attention to one little sentence

"with a portion of the proceeds going to the map's creators"

Meaning Blizzard will be pocketing a good portion of it. They are basically going to be making money off of other people's work without having to do much.

I'm telling you, this is just the beginning. Mark my words. You will not see a new Blizzard game that does not attempt to continuously take money from you (subscription fees for WoW aside) in some way. I would not be surprised to see a "Premium" subscription service for Battle.net, more microtransactions for WoW, paid downloadable mod maps for both Diablo III and of course StarCraft.

Activi$ion does this with every successful franchise they get their greedy little hands on. This is why we get multiple iterations of Call of Duty and Guitar Hero on a yearly basis.