Far more than commercialism Dave
The structure, sound and content of the songs and how they featured within the communities that made them their own identify them as what they are
We came to the folk scene drawn in by songs that had long been established as folk
- nobody went around with a rule book - they didn't have to - they sounded as they sounded and we took to them in thousands
People sang them, listened to them, wrote and argued about them - and went out and looked for more
Go through the lists - the BBC project, the hundreds of collections, the output of labels like Topic.... they are sogs of an identifiable type   
When you asked the source singers you got similar responses
Jean Richie summed it up perfectly when she wrote about her experiences in Ireland
"When you asked for the old songs you got everything from 'Home Sweet Home' to 'Danny Boy'.... then I sang them Barbara Allan; that's when the old folk songs came pouring out".
THey are of a related and identifiable type - nothing to do with the fact that some of them were commercially produced
Jim