Most of the time, though, I spend 30 seconds trying to get things lined up properly between my Nexus 6 and the sensor, then give up and use plastic instead.
Completely defeats the convenience of new tech.
Who's to blame for that cluster?
That's definitely up to the retailer (or more likely the gateway they are using). The one I use Apple Pay at most often used to do that and then switched to the more sensible no-sig policy.
It still sucks that you can't rely on it being at most places, though, because it means you still have to carry a credit card or cash if you're out for a run or bike ride and might want to grab a snack.