Ironically, it I difficult to argue with the NRA's right to oppose this law in purely legal terms - it does have a valid point about the restriction of the application of the Second Amendment.
That said, outwith the legal aspect, which is of course devoid of emotion, which it should be, the organisation is clearly motivated by financial gain, and the potential drop in its customer base.
I think there is an argument for asking why any citizen anywhere ever, wants to purchase a multi-firing weapon, since it is difficult to imagine a civilian use for a military-grade weapon which is designed to kill as many people in as short a time as possible.
In a military situation, that has grounds for use, but where in the name of sanity is that sort of weapon a valid choice for Zeke who shoots the odd rabbit for the pot?