See this article from Bankrate.com.
http://www.bankrate.com/brm/news/DrD...g_rules_a1.asp
The following is not intended as legal advice:
Technically, when cashing a check at the "drawee bank", you are making a "presentment", that is, you are demanding payment of the check which is a contract between the "drawee bank" the "drawer". The drawee bank has certain rights and obligations under the Uniform Commercial Code and other banking regulations. The drawee bank has until the close of the next banking day to either pay or "dishonor" the check. I guess we could interpret the fee as a payment for the bank's waiving its right to pay the next day.
Fellow attorneys: Please don't flame me, if I don't have this quite right. It's been a while since I took CP in law school.