What annoys me isn't that they ask for a tip, it's how much they ask. I'm willing to round up to the nearest dollar if the service was good, but those little iPads always seem to ask for at least 15%. I am not giving a 15% tip to someone who only pressed buttons on a tablet.
This was so weird in the US. Everywhere you get asked for a tip. I got a tip screen even in a supermarket once. For the cashier. I got back home to the EU today and was happy to not tip anymore everywhere...
It's the Square checkouts and the other new POS like them. I'm pretty sure Square takes a chunk of the tip which is why they're enabled by default (and I'm not sure you can disable them).