Something bugs me about airport parking

OK, here is something that has me wondering.  When I park at the airport (which is a fairly frequent event), I usually try to go through the automated credit card machine line.  That way I don’t have to talk to anyone, since I am usually not the nicest person after spending a couple of hours on a plane.  So when I go through that line, I don’t have to sign a receipt.  I have paid as much as $60 through that line before.  But when I have to go through the line with an attendant I have to sign, even if it is only $15.

WTF, over??

Mikey