You can get away from supply/demand laws. By pretending that something is not giverned by them and putting fixed price lower than real price (i.e. that price at which suppl equals demand) one leaves space for scalpers to exist and collect money that people are willing to spend over set price.

Given that people are going to spend money anyway it seems more honest and reasonable to direct this money to the party that makes the thing by letting it to run auction.