It's a very clever advertising ploy, but that's all. Results from this will be next to worthless.

1. Patrons of 7-eleven represent a rather specific demographic, which means this "poll" will have a heavy selection bias.

2. In a well-designed poll, each subject only votes once. Here, every time you buy a coffee, you submit a new vote. That doesn't work if one side drinks more coffee than the other side...