The rules on the boundaries of any given set of restrictions tend to be a bit contradictory, that just tends to be the nature of the game.

There are rules on, for instance, howitzers, and they are... legal... yeah no seriously you can own an artillery piece, fully functional. You can't just buy one, you do need a permit, but the reason no one makes a fuss about that is that you can't really hide a 155mm cannon's transfer, sale, storage, etc anyway and you certainly can't hide when one has been fired. They're not really a guerilla weapon... rocket launchers are... those are also legal, you just need a permit, they're not especially expensive or hard to get. Explosives are a bit different but that's like radioactive material, the stuff is genuinely a serious hazard to others to transport or store, also nobody much cares because making explosives is very easy, as is making mortars, making a gun or howitzer no. Several militia groups openly only own heavy weapons too.

Informally its decided by the current consensus of the people and how loudly certain groups are yelling, much like everything else. You can make a case that nuclear devices are not actually illegal as weapons for instance, but because they violate various codes on hazardous materials storage and transport, same as a guided missile needs to be cleared on account of FAA regs. Automatic weapons can accidentally keep firing when hot and can continue to shoot if someone clutched the trigger when injured, thus becoming indiscriminate. Those are generally the arguments made as to 'why X?' from a pro-gun but okay-ish with some limits on type attitude. Mostly though we're concerned about self-defense and guerrilla warfare options, automatic weapons are pointless in guerrilla warfare, though popular, because they waste tons of ammo for usually little purpose and that's assumed to be a very limited quantity in the $hit/fan scenario, ditto cannons aren't much use, too big, and the same applies to most of the bigger ordinance. RPGs are about the only particularly handy weapon we have restricted.

"Is the object a significant hazard to transport or store? If so, license" is actually about the long and short of it. As to why its treated as an attempt to remove them all, its not just legitimate fears of slippery slopes or how many 'civilized countries' have outlawed them entirely, which would be enough, its that a lot of the people who rant around about this stuff say we should ban them all... kinda makes trust and good faith negotiations tricky. We do know most people don't want them all banned, we just aren't clear on how any of the arguments brought up wouldn't apply to shotguns and handguns as well, with guys like the V-tech shooter proving mass murder that way is doable, and so we figure if they outlaw X, they'll just wait to some guy does it with Y then outlaw Y. That's not paranoid, that's what most of the people I was raised by and around wanted to do at the time and I'm not that old and people haven't changed that much. The rebuttal, that mainstream politicians aren't, requires a certain extreme suspension of disbelief, like the idea of an honest used car salesman, or the notion that there are no bigots in congress just because they never use slurs in public.