Not a law, but I think it's the CC-issuing company (i.e. bank) that says you can't charge extra for taking credit cards. That's why the discount computer places advertise prices that are "discounted 3% for cash or Interac" rather than "3% extra for credit cards" although it amounts to the same thing.
At EWR they give drivers tickets that control the order cabs can go to the termanals for pickups. If they get local fares they are allowed to go back to the front of the line if it is within a certain amount of time.
I think they do this at LHR too (where the lines can be 3hr or more). In fact I read somewhere* once that one scam is to tell the dispatcher you're going to a local destination, make sure he gives _you_ the slip for the short line on return, then after you've pulled away, tell the driver you're actually going to Central London and haggle with him for a break in return for the short-line card.
*I'm sure it wasn't on the BA forum on FT, oh no I'm sure it wasn't.
andrew