We live in a society where there are a number of unspoken grey areas where things are technically in violation of one regulation/law or another, but the police and other enforcement groups are expected to offer some leeway in order to allow common sense to prevail. 100 vs 120 in the highway is the most obvious example. They know people want to push the limit, so leave it at 100 knowing the bulk of traffic will travel between 110-120. This also leave some cushion for ticketing above 120, as the police aren't expected to make decisions about margins that fall within the error range of their equipment. It's a system that's flawed, but it works well enough.
If police were to start enforcing every single regulation on the books rigidly, it would create a social crisis, as laws would need to be updated so regularly it would be impossible to keep up. Nobody cares about those antiquated fines for things like riding horses into bars because it's seen as common sense to not get too fussy about it.
Some things need rigorous regulations because life safety risks are so high. Flight operations, for example. Reflectors on license plate hangers aren't on the same level, hence getting ignored.