While I agree that there needs to be oversight and investigation, I wouldn’t let a government official search my business without a warrant either. Are you going to let the ATF search your gun safe without a warrant just because they knock on your door?
Even trying create probable cause because the employee wouldn’t let them search is a violation of the Fourth Amendment.
Don’t get me wrong, I hate what’s going on with operations like the one mentioned in the article. However, I believe in fair and equal protection under the Constitution.