There have been routine searches of bags in airports for over thirty years, people entering most government buildings are also subject to search. As long as it is posted that any person or their belongings are subject to search, it is perfectly valid. People who have something to hide need not enter. Nobody is forced to use any form of public transportation, there is nothing "unreasonable" about conducting random searches of the people who do. If someone has illegal drugs or something else that is illegal but unrelated to terrorism, they have always been subject to prosecution if it is discovered in an airport or someplace else. The reason given in the early 1970s for searches in airports was the threat of skyjacking; however, people have been arrested and prosecuted for plenty of other things.
This is a non-issue, the ACLU knows it. The ACLU is concerned that racial profiling will be used to determine who to search at random. What the ACLU doesn't understand is that many (probably most) Americans are supportive of racial profiling when it comes to fighting terrorism. We know what terrorists generally look like, these are the people we should target.
What you are saying is false. Compelling random searches of individuals using public transportation is illegal, and therefore it is only voluntary.
The government has been using the Constitution for asswipe since the New Deal. Citing a bad precedent from the 70's means nothing. John Gilmore has only recently started a legal challenge to having to produce ID to pursue an entirely legal activity, without being in any way suspicious. Let's see if that document still means anything. If not, who cares?