I am not sure anyone has defended them anyway!
People have pointed out that is a teacher (or almost any worker) was found to be doing another job , by their employer, during working hours, they would be disciplined.
You then said they should be volunteering. Which, if they were doing during working hours btw, they would also be disciplined for, if it was found there were aspects of their role they weren't fulfilling.
I am not going to RTFT again but find me a post where specifically a teacher has defended a teacher's right to take extra earnings while not working under the instructions of their headteacher and discharging their role.