The same as for any copywrited work. There are companies that look for and prosecute illegal use of copywrited material. The laws and mechanism is already in place, just add NPS permits. You probably hardly even notice Used by Permission at the end of an attribution of some work.
What if we change the 'money making' endeavor from creative works of photography to canvas art, or how about physical conditioning?
Take for instance, an artist with a canvas, some oil paints, and a few small paint brushes, and they sit outside their camper van and paint the sunset they see. Or maybe the saguaro cactus, and a hilly landscape, and a cute little ham-can camper that's parked nearby. All captured on canvas, but looks as good as any scenic painting you have ever seen. Inspired by our public lands.
They paint a few of these and then go home, and sell them on ebay.
One of our members here is doing something kinda similar. Creating a product, while they happen to be parked on public lands. Commercial use, yeah maybe, but absolutely a VERY LOW impact.
Or how about a couple, they show up with bicycles and ride them for 20 miles a day all over the BLM or LTVA or national forest roads and trails....Physical conditioning, in a nice public area, and then they go home after a few weeks of physical conditioning and compete in a triathlon and get first place in mens and ladies, and then get sponsorship and start earning a small supplemental income, because they were able to train on public lands.
I mean, where, and HOW do you draw the line? As long as the 'impact' is minimal or essentially zero, there is just no need for a fee or a permit for that activity.
In the context of a US citizen on public lands, I see no difference between pushing a shutter button, hitting a record button, applying paint on a canvas, or pedaling a bicycle, they are all low impact and any attempt at imposing a fee or requiring a permit is government over-reach.