You're focusing on the wrong part of the story. The position of power, and the abuse of that power is what is such an outrage. Not just the crime per se, although that is pretty fucking awful on its own. So this dickhead thought this girl was cute, and thought he could leverage her freedom, against his authority. The man should be shot and killed in the street if this is true(which it may not be and I'll touch on that). Why ? Because of the crime ? No, because of the effect of the abuse on the communitie's trust in that authority. Police, judges, politicians and everyone else in public positions of power should be held more accountable for their actions than a normal citiizen, held to a higher standard, and pay more dearly for breaking that trust.
On to where and how this may not be true. So this girl has a warrant, and a record, so she may not be the most trustworthy character. Keeping this in mind, she may have openly offered herself to the officer, and being of weak resolve, he may have agreed to barter. However he may have never had the intention of, or later realized he could not keep his end of the bargain, and reneged thinking she had no way to prove her claim.
Either way the cop is more in the wrong. Just imagine if your wife or girlfriend, or hell your MOTHER was pulled over by some asshole for speeding, or more realistically incarcerating, being a hare over the legal alcohol limit after a night out with 'the girls'. How would you feel if you found out the officer was trying to mouthfuck your wife as opposed to doing the job we all pay him to do ?