The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated...U.S. Constitution, 4th Amendment.
They must have at lease reasonable suspicion ("specific reason to believe a crime is being committed") to stop the bus and need probable cause ("fair probability a crime is underway") to search and seize.
In this case if not a warrant then at lease probable cause.
Fine, get a warrant for wanted criminal “Juan Lopez” and check every damn bus to see if he’s onboard. Easy.
Yeah, it's "probable" that there are wetbacks on that bus.
(plus the passengers don't own the bus)
So what about the road blocks checking on any motor vehicle operators who may be dui?
Is it a one year lease? How about a security deposit? Are pets allowed?
This is public transportation. I’ve been searched many a time getting on an airplane.