Pop rivets are a very common way to pin a mag. Because they need tools, they are likely (I say likely because I cannot find in writing anywhere) fine. I have mags so pinned and I'd have no problem living with them.
It would be nice if the long tailed followers were OK too. Especially since the M308 shipped with them years ago. I can see that running afoul of the regs because all it requires is swapping a short tailed follower from the 5 round mag.
The rules don't have to make sense. The rules certainly don't have to work. They just suck. No mag capacity limits is what we should be shooting for next. Just think of the boost to the economy as we all a. buy new normal capacity mags, and b. spend more on ammo.