If a cop can't see through your tinted windows, that will get you a ticket all by itself in most states.
The window tint law in my state (West Virginia) is max 65% on front windows and any level of tint on back passengers and back glass. (which even at 65% is still dark enough that you can't see in unless you are only a few feet away.) It is legally set by percentage though, it has nothing to do with whether a cop can see you or not.
That is true. The law cannot state "if a cop can't see you," it's too vague. 65% is a pretty good number, you're lucky. The number is usually set so that you can be seen and ticketed for whatever they please...there is no other reason to have a law.