The problem is enforcement. As it is this law is all about tractability of enforcement, it doesn't change what apartments are or are not allowed to be let/sublet but rather adds a provision that makes it illegal to advertise an illegal unit. This is because the existing law is very difficult to enforce and AirBnB moguls are blatantly flaunting it. This is the same problem with those that suggest that we don't need state laws because condos and co-ops can just enforce their contractual rules. (Also, that argument neglects rental buildings.)

Relaxing the law in the manner you suggest would make it even more difficult to enforce than the law as it stood prior to this addition.