Airports also receive revenues directly from our taxes, at the Federal, State and Municipal levels.
I think the concern is not that the big FBOs charge for their parking spots and other services. It is more that even touching down at a small regional airport, which is also supported by our tax dollars, can incur significant FBO fees. I.e., you could not pick up/drop off a passenger on a public ramp. My son was visiting last year and wanted to flight see a bit. We flew to Cortez, then Telluride. At Telluride, we flew up the valley looking around, landed, and immediately taxied back for takeoff. My son took a photo of the lighted density altitude sign (it was over 12,000...!). As I was taxiing back, the FBO called to tell me I needed to taxi in(a long way), shut down, and pay the fees. I told them that I had not entered their ramp at all. They said that didn't matter.
There was no one on the CTAF for the 15 minutes as I was approaching KTEX. There was one bizjet coming up on the radio to taxi out as I taxied in. Not a "busy airport" that day.
I looked up KTEX's FAA funding from the FAA(i.e., federal taxes) for last year. It was approx. $1.4 million. Don't know what they received from the state of Colorado, and the city of Telluride.