Randfan, you're using circular logic. You say you don't accept the distinction between assault weapon and assault rifle, despite assault rifle requiring select fire or automatic only, because gun enthusiast use the term assault rifle. But you support assault rifle and assault weapon being the same by gun enthusiast (marketing departments actually) using the term 'assault rifle'.
Umm just so you know, while selective fire is a common feature of assault rifles it is not their defining feature.
The primary feature of an assault rifle is the use of a mid sized round, which unlocks a range of useful capabilities for combat. Rather than simply outlaw mid sized rounds it seems lawmakers are targeting those capabilities instead.
The logic behind a mid sized round is:
It's still accurate to ~200-300m. A sub-caliber round which are not usually accurate beyond 50-100 feet (sometimes by design, see below). While a full sized round can be accurate at much greater distances, it turns out that most combat takes place within that 300m range and in cases where it doesn't simply having lots of people firing lots of bullets can still kill the target.
It's smaller and lighter than a full sized round so you can have larger magazine capacities and carry more ammunition.
It has less recoil so you can fire a lot more rounds with high accuracy than you could with a full sized round. Even so in bursts over 3 round recoil is still going to have you firing well over your targets head. 50+ accurately fire rounds per min is probably within the capabilities of most assault rifles, though most marksmen can't approach that.
While semi-auto is pretty much a must, a full auto or burst setting isn't, but is usually included to allow for suppression fire and emergency use in very close quarter combat. Other weapons are preferable for the latter, but it's nice not to have to carry more than one.
Fully automatic weapons typically have a different job than an assault rifle. They are designed to spray massive numbers of bullets in the general direction of a target. In these weapons, accuracy is actually a disadvantage. They fire so many bullets nearly everything in it's kill zone is likely to be hit, but the more accurate it is the smaller this zone. The point isn't to hit the same target as many times as possible it's to hit everything in an area.
A machine gun would be such a weapon that uses a full sized round, while a sub-machine gun uses a pistol round and is typically intended for 1 person use in close quarters combat. While it would kill more people than an assault rifle in crowded area, the way most shooting sprees play out someone with a sub machine gun would run out of ammo long before someone with an assault rifle stopped killing people.
Neither are really good choices for other application. Both lack the accuracy and range to be really good hunting rifles, and their propensity to spray bullets around the neighborhood makes them irresponsible to use for self defense. I tend to agree with the people above who say that the main appeal of these weapons is to stoke the gun fantasies of their owners rather than practical application.