A MANUFACTURER can "re-manufacturer" a firearm. In doing so, they must engrave their license name and location (City State) on the firearm and pay any applicable excise tax (couple ways to calculate, but usually 11% of 75% of the retail of the firearm) The excise tax is a mess because, generally, you don't know how much credit to take from what the original manufacturer paid. The way the system is supposed to work is the original manufacturer transfers the firearm without paying any FAET so then the re-manufacturer or upfitter pays the full tax. There are also additional record keeping and form filing requirements.
Not that it can't be done, but quickly it becomes not economical to do and follow the law.
It sounds like the main purpose of this law is to prevent manufacturers from dodging excise tax by selling bare bones guns to dealers, paying a lower excise tax, and having the dealers add valuable enhancements, excise tax free.
From what I understand, if a buyer could legally take possession of a gun from the dealer, he could immediately hand it back to the dealer and pay him to do the same work (getting rid of threads or other evil features) as gunsmithing work. But it is a catch-22, because the dealer can't modify it without first selling it and the purchaser can't buy it without it first being modified. Perhaps a more pro-gun ATF under the current administration might change this ruling.