Why Project Expenses are defaulted to Billable although Engagement Expense Billing is set to "No Expense"?

Question
The Engagement Expense Billing option is set to "No Expense" and the project is billable and configured with the option "Allow Resources with task assignments to enter expenses". Why Project Expenses are defaulted to Billable? 

Answer
This is the design of application: this checkbox indicates that the expense can be billed to the customer. If customers change their mind later and want to invoice expenses, the expenses are still available.