> ...25 people who had tested positive for drugs were let go and not held. They were probably connected somehow, or paid a bribe.
There are plenty of perfectly rational reasons why one might test positive for drugs but not be held.
In the US, at least, your levels must be high enough that they are unambiguous when factoring in margin of measurement error, but any amount is enough to get a warrant or have just cause to search your person/vehicle.
Alternatively, I take a prescription medication which causes me to test positive (hilariously high actually) on some drug tests. I carry my prescription and doctor's note on a prescription pad whenever I travel, and it's not that uncommon.
Jumping straight to corruption is unfairly stereotyping China, which, though it may have a problem, has many more honest cops/prosecutors than dirty ones.