They do decide what's legal, but do you have an opinion? They're only human, and you seem to care about the Constitution a lot.
I think the framers made the fascinatingly poor decision to punt on the issue by banning, in the 4th amendment, "unreasonable searches and seizures" while leaving "unreasonable" open for interpretation, thus making it impossible to be sure that the first half of the 4th bans anything at all. It's actually even worse in principle than it is now, because the Constitution doesn't technically give the Supreme Court the power it has arrogated to decide that (and other issues of Constitutional interpretation), so it was just... completely left open.
(Obviously, I have my own definition of 'unreasonable', but I'm not even going to begin to argue that anyone else should be required to abide by it.)
Then again, in all fairness, the framers never intended for the Bill of Rights to apply to the states at all (that was added with the 14th, again not actually in the text but the Supreme Court decided it did), because they seem to have never conceived that the people would let their own states turn against them. So arguably, the intended scenario as it applies to the federal government may have been "the federal government can't do more in terms of searches and seizures than the states would". That may explain their failure to lay out clear limits, but, I'm a textualist, not an originalist, so it doesn't really
help.
If I were a Justice, I'd be employing the rule of lenity here and saying that the only viable conclusion is that the 4th bans searches and seizures which are unreasonable
to the defendant, but that's obviously a losing game because it would essentially make it impossible to search and seize at all (unless you can somehow demonstrate that the defendant found it reasonable to, say, be arrested).
I'm fine with that, but I'm an anarchist. It doesn't play well in Peoria. So if you want to be able to have a police power and stop people from doing crimes, the limits of the 4th are up to the courts, which, honestly, is usually a very good system. The main problem is that people apparently like no-knock warrants.