I genuinely believe that better than 90% of the time it has everything to do with the attitude (or lack thereof) presented to the cop(s) when you get stopped.
There are several things that are technically illegal on my car. The lights on the license plate have red bulbs in them as opposed to white/clear. Louisiana has a rather ignorant law about running driving/fog lights in non-inclement weather (and if it's dark, mine are on). In LA you're not supposed to have anything darker than 40% on the front sides, 25% on the back sides, and 12% on the rear window ~ all of those figures being NET light reflection rather than the percentage on the tint itself not including the slight OEM stuff that comes on the windows from the factory. I have 35% film on my front doors and 5% film everywhere else including the windshield "brow."
Do I think silly-ass laws like that are purely for revenue generation? Absolutely. It's also been my experience that the majority of cops are pretty decent people that don't want to hassle you any more than is necessary so they can get back to their busy schedule of hanging out in convenience stores/doughnut shops drinking coffee and running their mouths. Don't give 'em an attitude and more often than not, unless you have a criminal background and/or active warrants on you they tend to not really care what you do and don't do with your own car.
I've been stopped multiple times and been given the "too-dark tint" excuse as to why I was stopped. They've had a quick look through my car to see if there was any contraband, run my driver's license to check its status and mine, run the numbers on my concealed carry permit and that which it permits me to carry concealed, etc etc.
So long as you don't have a history with legal run-ins and you're paperwork is legit (current driver's license / insurance / registration / inspection if your state requires it, blah blah blah) they'll normally send you on your way with an admonition to "Be careful" or some such silliness. At least that's what keeps happening to me *shrug* Maybe I've just been really, really lucky?!