It is non-trivial to get dollars into accounts on exchanges. It is also not an instant transfer of bitcoins from one exchange to another. your two step process really is a four step one:

     0. Get dollars to Bitstamp to buy bitcoins
     1. Buy bitcouns from Bitstamp
     2. Transfer bitcoins from bitstamp to bitfloor
     3. sell bitcoins on bitfloor (assuming the price hasn't changed).

What you are describing is "location arbitrage" (exploiting differing prices for the same goods in different locations). and it is one of the ways that prices get moved back into line.