Signs of the times

At the exit to our local Quiktrip petrol station, there are two signs for exiting drivers.  The first says "Right turn only."  All well and good, though you could have worked that out from the way the exit points to the right, making it almost a U turn if you really did want to turn left, and the lack of decent sight distance to work out if a left turn was safe.  I have no issue with the first sign.  It's the second one that bugs me.

About two metres on from the first sign is one that says "No left turn."  I think that if a person can convince the authorities that they should be allowed to drive a car, they can work out that having to make a right turn precludes making a left turn.  Is it that Quiktrip thinks its customers are unusually dumb, or that they're scared silly about someone making a left turn, getting squashed by a big lorry they couldn't see, and suing them.  Probably the latter.  But if that's the case, are two signs really enough?