Magazines hold ammunition in a way that a gun can pull a round from it when the action cycles. Clips are a specific kind of device for loading some types of magazines.
I don't know of any examples of guns with this sort of setup that would get anywhere near 15 rounds, but I suspect that the M1 specifically has something to do with the common misuse of terminology. It may also be related to some of the loonier misconceptions about guns, such as the idea that "clips" (actually magazines) are disposable (and therefore somehow also not reusable) (example of this misconception: http://en.wikipedia.org/wiki/Diana_DeGette#Gun_control).
In the case of the M1 and the SKS, the magazine, eg the device that holds the ammunition inside the gun, is the restricted item. The only difference is the magazine is not removed in the field, like with most other guns where that's how you reload.