You clearly know this more than I do, and I'm probably wrong in general, as it seems. I guess I find it hard to believe ATC is just flagrantly flying planes full of people in airspace they shouldn't be, but maybe that's normal.
You’d be surprised! For example, when I take off from Palo Alto in the Bay Area and fly northeast over, say, Fremont, staying under 4000 feet, I am in what’s called Class E airspace where I am not required to transmit on the radio. I regularly see arrivals into Oakland in front of me at my altitude and below. It is technically considered controlled airspace, as IFR aircraft must operate on an ATC clearance, but my VFR ass can legally barrel right through without talking to anyone! Obviously that’s not the smartest or safest practice but it’s legal.
