I can't tell if you're being sarcastic but in case you're not, the question is not whether it should be forbidden (e.g. by park rules) to "wash a fish at a faucet if it's not a fish-washing faucet, in a national forest" but whether there should be a law that makes such thing a federal crime.
Putting "Do not wash fish in faucet" signs up would probably go a long way. If the problem gets completely out of hand, make it a civil offense that carries a fine.
Here's the thing, though: federal lands are administered directly by the federal government. The only avenues they have for rule-making are Congressional statutes and agency regulations. There is nothing else available to them to achieve the goal of making and enforcing a rule, because that's how they're legally set up.
So to forbid washing fish at a faucet in a national park... you literally do have to make a federal law (or a regulation with the force of federal law).
This is also why they have their own law-enforcement agency (the United States Park Police): Congress was, for a very long time, reluctant to authorize a general federal police force, so many agencies have their own specific police force operating solely within that agency's jurisdiction. It also creates fun inter-jurisdictional issues since the U.S Capitol building has its own separate police force, but the Capitol building is on the National Mall, which is Park Police territory.
According to 16 USC 551, a fine is indeed what you're likely to get for violating this regulation.
The issue here is that the statute authorizing criminal penalties covers a whole range of national park regulations, some of which clearly merit stiff penalties, and some of which are just about washing fish safely.
But is it a "crime" (one that gets you a criminal record) as opposed to something like a parking ticket? This is what I thought we were arguing about but IANAL and might have misunderstood what OP meant.
It's a misdemeanor and gets you a criminal record. But so does a speeding ticket in states like Maryland or Georgia where all moving violations are misdemeaors.
