This import ban is based on a claim that this cartridge is "armor piercing." Well, of course it's "armor piercing." Pretty much any centerfire rifle cartridge is, in comparison to common low velocity pistol cartridges, relatively good at defeating bullet resistant vests. Bullet resistant vests for law enforcement were originally, and remain primarily, a defense against common pistol bullets. Nobody ever pretended they were much good against common deer rifles until quite recently, when more sophisticated vests became more widely adopted. And these sophisticated vests will often defeat genuine armor piercing military high power rifle cartridges anyway -- a much harder task than stopping one of these medium power 5.45mm bullets.
So, it's bullshit. No other word for it, really. And this makes me wonder if there might be some other bullshit spread around that I haven't recognized.
PS: For those who do not already know this, the old Soviet standard 7.62X39 cartridge is far and away more popular with US shooters than the newer 5.45X39, and few AK rifles in the US are patterned after the AK-47. The vast majority of AK variants in the US are semi-auto only designs based on the AKM, which was a mass production redesign of the AK-47 and was introduced to Soviet forces in the 1960s. In other words, the 5.45X39 is an obscure cartridge here in the US -- and this has apparently made it an easy target for what is probably a carefully calculated test of the practical limits of Federal over-reach on the general issue of ammunition restriction. Ain't your bullets? Brother, they're all "your" bullets.