Probably because Burners will give other Burners a break and sell an extra ticket at cost, which he only gets because he's lying to folks.
I get what you're saying (lying is bad, lying for money is fraud, no argument there) but think for a moment about why scalping is a crime (or frowned upon) in the first place: because it artificially raises the price of something past the point where it can be efficiently used by the maximum number of people.
If your "kindly BM" theory is correct, and it may very well be, then there are people heading out to BM with extra $380 tickets in their pockets. $380 is a LOT of money (although, if we're being honest, not a lot of money to most of the BM crowd, aspirations to earthiness notwithstanding). But the ticket would be cheaper--and accessible without going through a scalper--if people like his sellers hadn't bought more tickets than they actually needed.
And if they hadn't encountered this guy, and made it all the way to the Playa with their extra tickets? They'd either have been wasted (artificially inflating the price in exactly the same way as would have happened if they'd sold to this guy), sold above cost (in which case they ARE this guy), or--very best case scenario--sold at or below cost on the doorstep of the event... probably to a scalper, who would immediately resell them at a profit online.
My point being, "Burners" selling tickets at cost are very likely motivated less by warm fuzzies than by the desire not to have pissed away $400 for no reason. Which is why scalpers can exist in the first place.