I'm no lawyer, but it seems to me that the courts have often inferred driving to imply consent to various kinds of searches that would be unconstitutional if you were just walking around. No person has to drive, so when you make the choice to do so, you consent to submitting to certain procedures designed to ensure the safety of the public. Don't want your property or person searched in this manner? No problem - take the bus or walk instead of driving.

EDIT: obviously I know that's not realistic for everybody, but it's the logic the courts have used to justify this kind of search.