I think there are several reasons for people not tipping wheelchair attendants:

1. Many people don't know they're supposed to tip. As someone else mentioned, we Americans are used to tipping but not always when to tip. Tipping waiters/waitresses and taxi drivers is a no-brainer. But we don't usually tip TAs, GAs, or FAs. And many people don't realize that wheelchair attendants aren't CO employees.

2. As someone who worked for tips while in college, I usually am a pretty liberal tipper. I feel that if I can't afford to tip, I should stay away from restaurants and taxis. But for those, there are non-tipping options (take-out food, eating home, taking public transportation). But for disabled individuals, getting to their flights isn't optional. I'm sure most of them would prefer to walk to their flights!

3. Most importantly, and overlooked so far in this thread, is that most permanently disabled (not broken foot or sprained ankle) individuals often are unable to work and thus live on Social Security (SSDI or SSI). And for someone living on S/S, that $5-$15 tip might mean the difference between eating for the next day or two! For a limited-income individual who already has the burden of high ticket prices (no disabled discounts from the airlines) and the cost of getting to the airport, this additional cost (which we non-disabled don't have to deal with) can be extremely burdensome!