What constitutes "unreasonable"? IANAL, but presumably, it would be doing something without having a good reason to do it, which is what's happening here. (Looking back, this looks sarcastic, but I didn't mean that sarcastically.)
Under your way of looking about it, that whole amendment is completely worthless, because "reasonable" is anything.
> balance the needs of the state against those of the people
There is no such thing as a "need of the state" that is opposed to "needs of the people." They're one in the same. I'm not claiming this is obvious, but I'm claiming that it's true. (BTW, there are no "needs of the people" apart from "needs of specific, individual people.")
Tangentially, your example of "avoidable" administrative searches actually is self-defeating. Air travel and road travel are completely analogous in every relevant way. So, the laws of the country are inconsistent.
Specifically, if commercial air travel is avoidable ("Hey, just drive from North Carolina to California,") then so is car travel ("Hey, just walk to work every day").
I mean, hell, why don't we just "avoid" going out of the house each morning?
The lawyers SHOULD go, "Hey, we can't stop cars on the roads without a reasonable cause, so we can't stop all air passengers, either." Of course, instead they'll just go, "Hey, now we can use the TSA to screen cars, too!" Talk about the erosion of rights.