If it's not federal law, then how does a federal agency enforce it? What in the hell are you talking about -- the BATFE "made that one up"?
Wow, you REALLY don't understand how this BATFE/FFL/gun-law stuff works - which is surprising for someone who spends so much time telling so many people they're all so wrong on the subject.
Federal law says what it does. This often does not, however, make a lot of sense - precisely because the people writing the law far too often don't know what they're trying to regulate, and don't know what the real-world repercussions of their words are.
BATFE is tasked with enforcing those laws. When the law is unclear, it is the BATFE's task to render "opinions" intended to clarify the unclear law (such as: what, exactly, is a "dealer"?). The BATFE then applies those opinions, and courts usually accept the opinions as if they were law.
In the case of "what is a 'dealer'?", the term "dealer" had been allowed for popular fringe cases where someone gets the FFL, and buys and sells guns in extremely low volume from his private residence. This was accepted industry practice, approved by the BATFE, for decades. Then Clinton and his anti-gun cronies stepped in, and wanted to eliminate as many gun dealers as possible; discovering that the majority of "dealers" (as legally & popularly defined) did not have a formal storefront, they got the BATFE to change the "opinion" of what a "dealer" is to require a storefront with posted hours - which caused the majority of legitimate gun dealers (licensed, engaged in buying & selling albeit in a small scale) to go out of business, as they didn't have the inclination or resources to obtain a storefront.
So yes, it doesn't have to be federal law for for a federal agency to enforce it. Yes, the BATFE is actually tasked with "making stuff up". This is the same agency that defines a "machinegun" as a gun they can, with extremely rare dangerously-ultra-sensitive ammo, thousands of attempts, and deliberate malicious manipulation of the inner workings, get to "slamfire" (not an uncommon malfunction) two rounds with one trigger pull (or put simply, they cheat). This is the same agency that literally declared a shoelace to be a "machinegun". This is the same agency that defines "pistol" as having exactly one - and not two - vertical grips. And this is the same agency that literally made up the absurd requirement that a "business" must have a "storefront with posted hours".
But no. You don't understand these things, even when people point them out to you, and deem yourself wiser nonetheless.