It's not true that amounts are entered right to left, rather what the cashier is typing is the value of the item in cents. If the cashier wants to enter the item price $12.50, they type 1, 2, 5, 0, from left to right. The advantage of entering prices in cents is that the cashier would not have to type the decimal dots.
What you are really seeing is simply the result of the mini display being right aligned. As a result, as the cashier type more digits, the digits previously entered get pushed to the left.
One of the reason the mini display are right-aligned is because many cashier machines uses seven-segment display, and usually the position of the dot is fixed in the display. If the display are left-aligned, the display would have to add decimal dots between all digits instead of just having one.