At least on the Delta side, they are not Delta employees therefore I will tip. Same with Southwest. There have been a few times recently where I did not have any cash and I couldn't give a tip (I don't typically carry cash because...no points). It was a bit awkward and I was debating whether I should say something or just walk away. I walked away feeling ashamed and my bag made it to my final destination. Oh well. Next time I'll give a bigger tip. I think the "tip culture" in the US is a little over the top. Tips should be for good service, and not to compensate for employers not wanting to pay higher wages. In other words, they are not entitled to my tip, however if they go above and beyond then I have no problem giving a generous tip.