Last year, US commercial air carriers bumped 40,000 passengers, and that's not counting those who voluntarily gave up their seats. United Airlines abruptly changed 3,765 travelers' plans with the involuntary bump, with 62,895 bailing out by choice, NBC Boston reports.
Read the fine print on any ticket and you'll see that when you board a commercial airliner, you are completely at the mercy of the airlines - they decide who flies and who doesn't. Period.
The US Department of Transportation can't make airlines change that policy, but it does make them offer stymied passengers compensation packages.