A simple tactile button costs around 10¢ of € on aliexpress, an ultrasonic proximity sensor is like 1.70€

So, unless there is a better way to understand the distance that i don’t know, they are wasting money, it’s just for the “modern and cool” factor to sell more

(All this without counting the price of a micro controller to read the sensor input, you could engineer the whole dispenser to work without one)

Edit: changed the price from 0.10¢ to 10¢, no ones sells things at a cent of a cent!