Why are Las Vegas cabbies so weird about tips?

[NajdorfDefense]

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I took a taxi from MGM to Paris and the driver took some back street and we got there super quick and the fare was like 7$, so I was preparded to give him a nice tip. I then hand him a $20 and he fumbles through his money and hands me what appears to be 3 or 4 1-dollar bills and says "Is that good enough?" ... He then fumbles through his wad of cash and finds 1 more 1-dollar bill and says "this is all i have for change".

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See, now this is completely unacceptable and the mark of a real d-bag cabbie.