The point of pulling you over for lacking a front plate is to motivate you to get one. Clearly, pulling you over the first time wasn't convincing enough, as you did not get a plate. Also, cops don't have a hive mind. The next two to pull you over probably didn't know beforehand that you'd already been pulled over for that. If the frequency of this is bugging you, get a front plate. "I keep breaking the law and it's so annoying when the police try to enforce it."
Cops aren't "dicking" with you. It's not personal, it's their job to enforce the law. Get over yourself. To a cop, you're just some guy in violation of the license plate law. Not their fault that you chose to ignore the first pullover and continue flouting the law.
Also, enforcing minor things strictly sends a message that they will pursue major crimes just as much, if not more. The basic idea is to give people an idea that since minor crimes are so hard to get away with, major ones will be even more so. Sounds kinda funny, but it has been shown to work.
Technically it's not "abusing" the ordinance, it's enforcing it. What is the point of a law if it is not enforced? It may as well not exist. Why have a law if you're going to let everyone get away with breaking it? Unless they are framing innocent people, they are not over-enforcing the law. Enforcing the law is their job, the point of the existence of police departments.
It seems to me you keep treating this as if it's some personal thing against you. It's not. The state or city governments make laws, and the police forces' jobs are to enforce those laws. If an officer is just on patrol, not on his/her way to a crime scene or anything, why wouldn't they enforce minor laws? It passes the time, brings in revenue, and enforcing the law is what they're paid to do.
If you think the law is unjust, work on changing it. Complaining about the police doing their job is not the way to do that. And, it can be done. For example, the citizens of Santa Cruz, California, voted to make marijuana enforcement classified as the lowest priority of the Santa Cruz Police Department. What the local measure basically did, was effectively make the police unable to bust people for marijuana use unless they were already getting busted for something else as well. This course of action was much more effective than people griping about getting busted by the cops for smoking weed.