We all know what the laws and rules are, and it's not all that difficult to avoid interaction with the police.
This is the problem - it's not true. You can't lead a normal life and simultaneously guarantee no interaction with the police. If you're the one guy on the freeway actually obeying the speed limit, more power to you - but it's abnormal behavior. If you've never had a taillight burn out while driving, forgotten to wear your seatbelt, crossed outside a crosswalk, rushed a yellow light, turned into the far lane, forgotten to signal a lane change, rolled through a stop, loitered under an awning during a rainstorm, not fed a parking meter enough change, eaten while driving, had your muffler crap out, annoyed your neighbors while barbecuing, dumped a finished cigarette on the ground, walked on the grass, drank alcohol before 21, smoked a cigarette before 18,
et cetera, et cetera, again, more power to you. But you're fairly abnormal.
Moreover, even if you are avoiding all those minor pitfalls, you can still be stopped for no reason. One spring break I was back from college, and hanging out with some old high school buddies. We went to a Denny's for dinner, and when we finished, we took a walk up the street the Denny's was on. We were trying to avoid taking up a table in the restaurant after we were done (they were busy), but we were still talking. On our walk back to our cars, we got stopped by an officer turning on his strobes and asking for all our IDs. Sure, he was polite and professional, but since when do you need an ID to walk down a public sidewalk? How should we have avoided that interaction with the police? Note that we hadn't even set foot on any private property. But three guys walking fairly aimlessly down the street around 11:00 was enough to get a cop curious.
Or how about the time I was waiting for a friend of mine at an Arby's. It was a hot, muggy day, and my breathing has never been all that good. I stepped inside the Arby's at one point, used the restroom, and stood in the lobby for a couple minutes, letting the air conditioning work its magic. Then I went back outside, and sat down on the curb. Ten minutes go by, and a police officer shows up with his lights on, and tells me he got a call from the Arby's manager about a suspicious person hanging around. Now, I suppose you could make a case that I was loitering for the few minutes I was in the store and not ordering, but I submit to you it's a bogus claim. It was less than five minutes, and then I was back on public property, waiting for my friend. This got me an interaction with the police. How should I have avoided that one?
This is what people object to, the fact that you have no control over whether or not you interact with the police. You count on their good intentions to protect you from undue hassle, and, generally, you're in good hands. But that doesn't make it any easier to accept that you can be hassled, and you can be grossly inconvenienced, and there's nothing you can do about it.