I will remind everyone of this great Milton Friedman quote:
The justification offered is always the same: to protect the consumer. However, the reason is demonstrated by observing who lobbies at the state legislature for the imposition or strengthening of licensure. The lobbyists are invariably representatives of the occupation in question rather than of the customers. True enough, plumbers presumably know better than anyone else what their customers need to be protected against. However, it is hard to regard altruistic concern for their customers as the primary motive behind their determined efforts to get legal power to decide who may be a plumber.
via Coyote Blog.