Without probable cause for a crime, they need to let you go, yes?

Sure, in an ideal world, but in reality they can always come up with probable cause. All they have to do is say they smell something illegal, even if they don't. They can make up whatever they want, and there's not much one can do about it.

As long as they don't get fired or jailed for even murder, cops don't "need" to do shit at all. If you survive, you might get free money for your experience.

The term among lawyers for this is "you can beat the wrap but not the ride"

You can be convicted of resisting arrest, with no underlying crime.