That is a specific law, I belive federal.  
Vendors are not allowed to pass on fees from 
certain forms of payments, particularly credit 
cards, onto the customer.  If you are going to 
accept that payment, you have to swallow the 
cost, or raise your prices for everyone.  I'm 
sure there is nothing they can do on an 
individual level like that, but it still is 
something to think about.