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Free market economics is a mathematical model that simplifies reality. It carries some validity under certain conditions, i.e. price is the prime motivator, there is a large number of suppliers and buyers, the product is not complicated and it's easy to change supplier. In the case of pot, where the prime motivator is recreation or addiction rather than price, then no free market economics doesn't apply as much as in some other situations. "
Free market economics doesn't assume that "price is the prime motivator." Demand comes from a lot of motivations and utilities, and it responds to price. For example, free market economics can be used quite successfully to analyze the sale of televisions, which is also a situation where the ultimate utility derives from entertainment.
Actually, utility of a good NEVER derives from mere price*. You're just confusing concepts.
*Except, of course, in a local sub-market where the price becomes too low with respect to a larger context, when something can be bought and sold at a profit. This is really still a different thing from the utility function that ultimately drives value, however.