Their very existence is an affront to the Bill of Rights. After 1968 Gun Control Act, firearms dealers were Federalized, so that you could no longer deal in guns like a regular business, but had to get special permission from an out-of-State, centralized "authority", regulating firearms sales in one further major increment.
The emotional charge to justify the 1968 GCA was largely based on the claim that Lee Harvey Oswald ordered his Italian Carcano 6.5mm rifle through the mail, and the new GCA would help prevent lone gunmen like him from having access to firearms in the future. The ATF's role as a punitive, confiscatory, regulatory agency was ramped-up significantly after that, with surprise inspections, FFL licensing, and the massive wave of seizures of FFL dealers' personal and business assets that began to erupt in the 1980's, and even more so into the 1990's.
Whistle-Blowers within the ATF frequently reported that their corrupt supervisors instructed them to approach cases with the following, easy-to-remember mindset: "Always Think Forfeiture". These same mid-level supervisors were commonly embroiled in blatant criminal acts themselves, to include adultery with co-workers' wives, fraud, falsification of documents, theft of critical evidence, embezzlement, waste & abuse of Federal budgetary resources, and professional destruction of any subordinates who called them on their crimes.
ATF relies on sourcing good, field-level investigators who come from mostly other LE agencies from local and State agencies, who go through a lengthy application and screening process for Federal employment, often taking 3 years or more to go through the process. Many local cops get paid a pittance, and deal with a revolving door justice system, where they see the same perpetrators arrested time and again, with a generally disaffected populace who mistrusts or doesn't respect their services, when the courts and prosecutors really deserve more of the responsibility for repeat offenders going free.
From that climate, an opportunity to get paid Federal wages, with Federal benefits, and make over $100k per year after a few years is hard to ignore for the professional investigator at your local LEA. He or she knows they will be putting up with BS either way, so they might as well get paid for it. Combine that with a spouse or ex-spouse with heavy monetary demands, or just the increasing costs of living, and the risks that LEO's take seem justified by going into Federal LE with the ATF, especially when the competition for USSS, Marshals, FBI, DEA, DOE, DSS, etc. is so steep. Once you realize you're being looked at seriously for Federal employment, you don't turn down any offers, unless you have the strictest of moral and ethical guidelines that would steer you away from the ATF.
Many agents who believe strongly in the Second Amendment can see justification in the BATFE because they frequently encounter criminal gangs who of course have illegal weapons, and many of those gangs are composed of illegal immigrants from Central America, Asia, Africa, and the Middle East. They solicit the assistance of FFL dealers in tracking these types of gangs, and honest business owners with traditional American values are happy to help out...until some mid-level supervisor orders his field agents to take down said FFL dealer because the manager is looking for monetary & property seizures, prosecutions, and sentencing....so all of a sudden, their 4473's weren't alphabetized, or a customer was mistakenly allowed to get away with abbreviating "Road" in their address. Simple clerical errors can have life-changing consequences for an FFL, and you will have no assets to hire an attorney with because those assets have been seized...leaving you with a government-appointed attorney.
The end result is intimidation of every FFL dealer, since they only need to make a few examples in the various Federal regions every so often, and the story will spread like wildfire via the pro-freedom groups, while any mention on the propaganda pundits will refer to the accused as guilty already, swaying public opinion in one swoop.
Agents within ATF have watched this type of crap go on for years, and remained quiet about it because they don't want to be personally destroyed, so they formed Clean-Up ATF.org
CUATF.org