No one would ever get heat from local law enforcement. Unless you have some crazy local laws. It's a FCC matter and enforcement is by the FCC at the federal level. And yes the chances of getting caught are slim...unless you're interfering with someone. Keep in mind a lot of amateur radio operators take part in "fox hunts" for fun and a fox hunt is just a matter of one guy hiding a transmitter and the rest trying to find it. In other words tracking down someone causing interference is a form of fun for some radio guys. They can't enforce anything either but if you're interfering with them they would inform the FCC and the FCC may or may not take any actions. If they do take actions you're looking at fighting a federal agency with a variety of enforcement actions available to them ranging from a polite "please stop" letter up to fines and potential jail time if you keep causing interference. You're also likely looking at loosing the option of getting licensed and becoming legal as one of their favorite enforcement actions is to deny future licenses to those caught causing interference.
Maybe a little visual example will help:
A 5.8ghz wifi router is like this:
View attachment 15071
A 5.8ghz fpv transmitter is like this:
View attachment 15072
It's a matter of power level. You may not annoy your neighbor by shining a little keychain sized LED flashlight at their bedroom window at 3AM. But if you were shining a 5,000 lumen LED spot light at their window they'd probably get upset.
And yes, the power level difference between a 5.8ghz wifi router and some of the 5.8ghz FPV transmitters being used is that dramatic. (though again the power level of the transmitter is only part of the equation, the efficiency of the antenna is a very big role.)
The big problem with this kind of "It's easier to ask forgiveness than permission" mindset is that in this case it's really not. As I already mentioned one of the go to enforcement actions the FCC uses on people is to take away their ability to hold a legal license. So if you do get caught (as unlikely as it may be) there's a considerable chance that getting legal won't be an option - and you'll have more scrutiny in the future as a past offender and a considerably higher chance of getting caught again if you keep doing it with correspondingly higher risks of facing stiffer penalties.
While on the other hand getting licensed legally is cheaper than what a FTV RX/TX costs, is a one time fee, and requires generally just a few hours of time taking practice tests to learn what you need to learn.
In other words "Seeking permission" is considerably easier and cheaper than "seeking forgiveness" if you get caught no matter how slight the chance of getting caught may be.