If flying is "a privilege," what about public transportation?

To all of the commenters (here and elsewhere) who say that those of us who are anti-scanner should stop flying and shut-up, because it's a privilege for us to be allowed to complete a lawful private contract we made with an airline: do you think it's in line with the Constitution to have random searches on buses and trains?

As I wrote over at LewRockwell.com:

A couple of years ago, there were threats of random searches of bags for DC Metro train riders (a la NYC and Boston), but I never heard anything more. Today, the TSA started random checks of customers’ bags and this apparently affects both the rail and bus lines for the DC area. It’s good to hear an uproar about this on the MSM, as they point out that there are petitions being filed against this action. But the quotes from the Metro officials are status quo:

“It’s good to vary your security posture,” he said, noting that transit agencies in New York, New Jersey and Boston have successfully carried out random checks.
…
The screening will be conducted before passengers pay to enter the rail system or board a bus, and customers who refuse the inspections will be “free to leave,” Taborn said. But there is a possibility that those who decline screening will be questioned.

Translation: We like to keep law-abiding citizens on their toes. And if they show resistance to this tyranny by peacefully walking away, we reserve the right to make their lives miserable.