Because what is banned is the magazine; pre-ban mags are legal in post ban guns. You can check it out under Title 18 of the US Code, that is, if you hae a week and a half with nothing to do!.
an important exception to the above "rule" is purported to be that what is "banned" is the entire magazine, i.e., follower, spring, locking plate, floor plate and tube.
there is a lot of commentary on HKPro (unfortunately his site is now down) in which it is reported to be legal to take a 13 round pre-ban magazine, acquire a post ban mag tube and spring and locking plate and floor plate, and use the follower from the pre-ban mag, as a result of which since "no new high cap mag feeding device has been created," the resulting 18 round (or whatever) magazine is in fact legal.
In any event, I'd not worry about carrying a pre-ban mag in a post ban gun. On the other hand, I'm not a lawyer and this is not legal advice.
tony