Well, I'm the head of this family and this family supports the republicrats, so I'm going to be looking at your phone honey, to make sure you don't make a mistake.

Or, I'm offering $10 to you, junkie, to support my party. But if you want to be paid I'm going to use your device/private key/one time code to do it myself. Or maybe you're my employee and we'll have a voting party where I can see you all wisely agree with me.

The privacy of a voting booth where neutral officials who are supervised by all candidates doesn't allow either of these scenarios work.