I know what you are implying and you may well be right, but we HAVE to report these things regardless, if you know for a fact that vehicles are uninsured and not MOTed or road taxed or whatever. Then at least you'll know what you are dealing with and can make a formal complaint if no action is taken by whichever is the appropriate authority, be it the council or the police.
You can't just sit back and grumble or assume that some sort of two tier system is being applied to people who flout the law, if you aren't actually reporting it anyway. That's just defeatist and does nothing to solve the problem. The law is the law and if the police fail to uphold it when something is reported then they are accountable.