Because, just like any business, the cost is always met by the customer. If you are increasing the cost to the business it WILL be passed on. The choice becomes how that is paid for. Either by a general increase across the board so everyone takes the hit or add a premium somewhere. Many businesses will charge a premium for specific additional services. IF we want the railway to act like any other business then we must accept their business practices too.
The customer ALWAYS ends up paying.
Appreciate that if a "Sunday within the working week" deal is agreed with an associated increase in salaries etc then that will be passed on for future Sunday services. What should not be passed on is any payments that take place now to ensure the services that they are supposed to deliver now are actually delivered.