Small blind tiny buy in
 

I was playing $20/$40 at the Commerce on Tueday night. The big blind goes all in on a hand, and loses. His small blind comes up, and he buys in for $10, just to cover the small blind. After his blind goes by, he buys in for a normal amount. I didn't think that short a buy would be allowed, but nobody said anything. Are you really allowed to do this, or is it just something nobody really wants to complain about?