I've gotten an answer on this directly from bter.

Turns out it isn't a direct restriction.

The problem is that a number of users have reported that the transaction is very slow, as a result of the mining pool not including a tx fee and having a lot of activity. But it does apparently work, it just might take longer than you might prefer or expect.

Rather than dealing with users complaining about their deposits not showing up, they just tell you not to do it.