Why? Because they can get away with it,

 and that's the fault of both the corrupt asshats running the BATFEIEIO and Congress.
Incredibly, in the hands and the realm of the ATF, an effort to clamp down on the power of the administrative state has ended up making the administrative state more powerful than ever. There are excellent reasons not to want federal bureaucracies to circumvent the APA. But the alternative to preventing circumvention of the APA is not private letter rulings. It is law passed by Congress and administered by the federal government publicly, fairly, and in line with Congress’s intent.

Anyone who says that the firearms industry just hates the ATF because the ATF is reining in the cowboys by private letter is, frankly, ignorant. The firearms industry is already one of the most heavily-regulated in the United States. Anyone who works in this industry and doesn’t follow the rules can get in big trouble very quickly. What the firearms industry wants is for ATF policies and interpretations to be open and transparent so as to promote an even playing field.
Anyone who thinks the asshats  will be 'open and transparent' without the choice of that or being unemployed- and possible prosecuted- is dreaming.