Jerry said:
"time is a finite resource and the work I do is ongoing."
Er...in what way is that not true for those of us who write? 

I have 'given away' a story to 100 Stories for Queensland, an anthology being published  on 8th March with all profits going to the flood fund. I did so gladly and one day I might do it again, either for promotional purposes, another charity, a favour for a friend...who knows.

But like anyone else spending time using their talents, experience or qualifications in a time-consuming job, I would generally expect to get paid if the product or service I offer is wanted by someone. After all, I don't expect my plumber to work for free just because other plumbers have fixed the same problem before...