With this particular law it is, actually. The problem is that the class of weapons banned by SAFE Act, and other similar "assault weapon" bans, is fuzzily defined, but more importantly, that definition doesn't have any rational explanation. An Australian-style full semi auto ban is at least justifiable on the basis of increased lethality, and there is an objective difference between semi-autos and manual action. Banning "military style" rifles with features that are mostly or wholly cosmetic does nothing useful whatsoever.
To remind, the firearm used in the single deadliest mass shooting spree to date - the one perpetrated by Breivik - was done by a firearm (Ruger Mini-14) that is not considered an assault weapon under any existing or past AWB laws, nor under any AWB proposals on either federal or state level, that I'm aware off. That alone should tell you all you need to know about those laws.
Why it's a slippery slope? Well, if you can enact a ban without a rational justification for the list of banned features, then that list of banned features can be extended arbitrarily in random directions at a whim.
Worse yet that these laws are usually written by people who have no clue about guns, and so they e.g. ban "barrel shrouds", and define them in such a way that practically every forend on any shotgun or rifle manufactured to date would be considered one (and so make it banned). That's not a hypothetical - it actually happened with an AWB bill that was proposed in Washington State this year:
"A shroud attached to the barrel, or that partially or completely encircles the barrel, allowing the bearer to hold the firearm with the nontrigger hand without being burned, but excluding a slide that encloses the barrel"
This definition practically implies that it's illegal to shoot a firearm while gripping it anywhere around or under the barrel. Makes you wonder if the person that wrote this have ever shot a rifle or a shotgun, or at least seen one being shot.