Why are Las Vegas cabbies so weird about tips?

[Maple Leafs]

I had a cabbie in Vegas spend the entire trip telling us how bad he had it, how everyone tips the dealers and the maids and the doorman but never the cabbie. How the guy who puts your suitcase in the cab's trunk gets a bigger tip than he does, etc. He even told us it was his birthday.

When we got there, the meter was $9.50. I gave him a $20 and he says "thanks", and I'm like "um, how about some change, bud?" He honestly looked like he was going to hit me. It turns out he thought I'd given him a $10. He was pretty cool about it afterwards, but for a minute I really thought I was going to spend the night in his trunk.