Is there a reason QM will let me sell a security in an account where it's not held?

I discovered by accident today that in a retirement or brokerage account, I can enter a transaction to sell shares in a security that's not held in that account. The security I entered the transaction for is one that is held in another account, but I discovered I can also sell securities I don't hold in any account without QM (6.1.1) batting an eye. 

I'm a fairly basic investor and don't do stuff like short selling -- which has its own transaction type anyway, separate from "sell" -- and can't really imagine why this would be a feature rather than a bug. But maybe there's a reason a more advanced investor would want this. I'd rather get a warning about the mistake I'm making.