From what I understand, you have to send the mags with the rifle that you want to use. Both the rifle and the mags are modified. They will still work in an M-1A, as per the law, but I don't know what the actual mods are, so I don't know if functioning could be affected. They recommend you send all the mags you intend to use, so they can make sure they all work.
If you don't already use an M-1A/M14, then it's no big deal, and you get a well built mag for a hi-cap that the VEPER doesn't have. If you already own a M-1A/M14, then I'd want to find out just what the mods are, and how they affect use in the M14 types. You still aren't any worse off than a non-M14 owner in having to buy mags.
Look at the mag capability as just giving the VEPER a hi-cap, rather than the other direction of another gun that uses M14 mags. Consider cost in that way.
Ross