Because at least 2 of the 3 healers will use it and no one wants to watch mass sleep DR wars. At the moment it is limited to purely FCs, and we'll be reviewing that soon.
While the BM ban is still to be tested, I disagree on the melee issue, because both teams are slowed exactly the same with BM. You're watching both teams waddle to places. With this change you might actually see movement and kiting become a factor in a team's strategy/gameplay, which is a good thing.
Either way, you will not be seeing BM in the 3v3 tourney because no one wants a repeat of the 19 3v3 with brew/mage/disc mirrors for 90% of the games.