If you want to go out and register folks in just one party, that’s fine and legal (and the way the electoral game is played).
If you pretend to register folks from both parties, then throw away the registration forms of all but one party — that’s fraud. It’s also something that should get you bitchslapped and tossed in the clink. And if it’s actually being paid for by the political party in question (the Republicans, in these stories) I hope they get fined until their eyes bleed, and the folks responsible face felony charges.
More details here, here, and here.
In the meantime, if you still need to register, don’t rely on some unknown person or persons sitting in front of a supermarket to take care of the paperwork for you. Do it right.