How is this any different from javascript validating input forms? The backend clearly verified the input, meaning the client would not end up paying the wrong a

How is this any different from javascript validating input forms? The backend clearly verified the input, meaning the client would not end up paying the wrong amount due to change of conditions or discongruency between the server and client. Exceedingly smart and simple, and the objections towards it demonstrates the modern cargo culting of "let the server handle everything it always knows best".