Maybe the rider wasn't under the influence of anything but was extremely tired and the stop made him realize he was in no condition to continue. Or maybe the stop made the rider realize that even though he technically wasn't under the influence his riding was impaired enough to have been noticed so the he decided to park the bike.
If I was ever stopped and given a sobriety test I sure as hell would not get back behind the wheel unless I was stone cold sober. There's no way I would press my luck. Plus I fully expect a cop will follow anyone they just gave a sobriety test watching for them to screw up again so they can pull them over again.
First of all, this whole (scenario) is full of problems...
1. If the officer gave the rider FST's *Field Soberity tests*, and the person was not arrested, then the rider should be allowed to leave.
2. Would you park your vehicle and take a cab home? Why? Makes no sense. What would the rider do if they came back and found their vehicle gone, vandalized, damaged? Would they blame the cops, you bet.
People get pulled over all the time for bad driving. At night, we look for DUI drivers. Just because someone is pulled over, and given tests, does not mean they are drunk. The tests are completed, and they drive away. Not everyone stopped is drunk.
There is no logical reason for a sober, meaning not impaired, driver to leave their vehicle and take a cab home.
Lastly, there is no reason to stop someone, check them for DUI, then allow them to leave, only to stop them again. You're comment about "fully expect a cop will follow anyone they just gave a sobriety test watching for them to screw up again so they can pull them over again." is disheartening. Don't lump a few bad cops in with all the good ones.