I think that you read Adams too narrowly. The court repeatedly stated that the FS cannot charge people “solely for parking.” Like any case, there are factual distinctions between the fee in Adams and the NWFP. And it would be up to a federal court in Oregon or Washington (or perhaps D.C.) to decide whether those factual differences matter. At its core, though, Adams held that the FS cannot charge users who enter an “area” (even if the area is within striking distance of the standard amenities) if the charge is “solely for parking.” A district court faced with a FLREA challenge to the NWFP could easily feel obliged to follow Adams, notwithstanding factual differences.
In fact, that’s exactly what happened in Fragosa. The FS made the same argument that you’re making: that Adams was about a fee program in a Colorado forest and, therefore, didn’t apply to a different fee program under different facts in California. The Fragosa court didn’t buy it:
“Adams is quite clear. The Forest Service is prohibited from charging a fee solely for parking. If a visitor does nothing other than park, the fee is solely for parking and is, therefore, plainly prohibited by the REA. Adams, 671 F.3d at 1143-44.” Fragosa.
Now, that doesn’t mean the NWFP would definitely go down. The Wiechers court didn’t feel bound by Adams, but the Fragosa court did. So we don’t know what would happen if the NWFP were challenged. But there’s definitely a reading of Adams that would cause a lower court to invalidate the NWFP.