No I have only ever lived in the US and I have used such systems on the employee end and recently which I suspect you have not. The pinpad universally doesn't go into payment mode until the transaction is set to complete. Depending on timing it is possible on some systems to go back to the prior screen and make changes and have the first tap still count e.g. cashier totals and selects card and the pinpad goes into payment mode, the goes back and adds an item then returns to payment mode and the transaction clears but it never not ever goes into payment mode initially. Think about why.

If you have no transaction what are you paying for

If the payment screen is up as soon as the first item is added how would you show any other screens e.g. loyalty number, donation prompts, ads and other BS.

If the customer taps then verbally says they would like to use a different card or pay cash how would that be implemented? The exact total will be oft unknown until the transaction is fully rung up and people make late transaction changes literally all the time.

People insert cards early all the time which is fine because it sits there doing nothing until the end they also tap early which does nothing but because they are extra stupid this doesn't hurt much

They don't tap long enough close enough or in the right place on their card or on the machine so to them thry are already just molesting the machine repeatedly with the card until it beeps anyway.