I too would like to know what pre-existing social structures are, exactly. Free market capitalism does work. It needs to be regulated to some extent, but it works better than any system in human history. And I think you're essentially wrong...the vast majority of "human interaction" IS economic. Money makes the world go 'round, as they say.
The genetic proclivity to associate with others throws a wrench into *purer* free market capitalism of the sort RP is a proponent of. Extending from there to tribes, cities, nations all with borders and super social obligations, free market capitalism ONLY works when balanced against these social structures that came to exist far before the Scotsman had his laisez-faire insight.
You are a teacher right? Does your instruction serve an economic purpose, directly? Of course not, these children aren't paying you to teach them. At best their parents are, through some sort of property tax dilution. But why the fuck would parents want their children to waste time getting an education, if every interaction is economic? They are far more valuable squeezing oil from their loose change. If parents own their children, and if everything is economic, they do, why would they pay YOU to teach them music, a generally useless skill set? Hell, why don't more children kill their parents to get that yoke off their backs?
Economics is a minor aspect of society. Family, friendship, and other non-economic interactions dominate our day to day lives. We are genetically social creatures, not economic ones.