Err... the second line of my post....
As for why SQL's using the memory, because it can. Because it has not been told that it can't (via max server memory). Set max server memory and have a read through chapter 4 (which is around 15 or so pages) for all the details on what to set it to, how SQL uses memory, queries that let you view memory usage, etc.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability