he’s right. they won’t “lose” them. they will have them “taken from them”.
Think about it. With a ten round mag you would have to change out three times to shoot 30 rounds. This give you practice at the range in quick changing them.
No one will turn in or register (how?) their regular 30 round mags anyway. They will just keep them at home for when SHTF then it would not matter then.