When you add a time log to a ticket, the "Billable" option will be enabled by default. If you are looking for the time logs to be applied as non-billable by default, you could make use of the “Non-Billable by default” app.
Installing this app will make any new time-sheet or time log entry created for a ticket as “Non-Billable” by default. With this app, any ticket that requires to be billed will have to be done intentionally and you can prevent your agents from creating a billable time-entry by mistake.
Please refer to the link below for more information -