I think it's a question of race and class. Most of the skycaps seem to be overwhelmingly black and the travelers who use them are mainly white and I think the skycaps resent this. The first time I used a skycap he reminded me in a very nasty tone that I had forgotten to tip him. Really I had no idea I was supposed to tip. I didn't have any change so I gave him a $10 bill I was so intimidated. Skycaps look very unhappy.