Laws, rules, and regulations (and the legal interps of them) change all the time. Sometimes more loosely, other time more restrictively.So what. They shouldn't be able to change the rules and go back on their word….
The real beef with this particular rule should be that both the content and manner in which ATF promulgated the final rule violated law. When any government violates the law in thru the exercise of power, that’s the deepest breach of the social contract that can occur.