One would think that there is some documented history concerning this provision of law. Whether or not such documentation can be found, it seems likely that free access to public lands was one of the many assurances provided to quell early opposition to the general notion of a national forest system. If nothing more, the idea of charging general admission fees is a radical departure from the early intentions of the creators of the national forest system. I hope that I've captured the main themes. Fees are a direct tax for use.
People ought to pay for what they use.
The US Forest Service has always relied on fees as part of its funding. The agency was set up that way. Environmental Justice problems loom large when recreation fees are charged. In the main two fundamental American traditions, and the values that underlie them, collide when we speak of recreation fees charged for general access to the national forests. We are embedded in a culture founded in part on principles of capitalism, private property, free enterprise, choice through the marketplace, and individual rights to use what one can pay for subject to prohibitions imposed through law and custom.