Simple justification: limit the number of people in these industries by creating an artificial barrier of entry. Which is a necessity for every industry not having a natural barrier of entry. Otherwise, things net nasty.<p>For a good example of what a non-scalable industry without a barrier of entry looks like, go to every freelance programmer marketplace. You can compete in such a marketplace in just one way: working dirt cheap AND scamming your clients. Not a win for either freelancer or a client. If you needed to pay $1000 a month for being on the freelancer site (imagine there is just one freelancer site out there, i.e. we are speaking of cost of a license to be a programmer), things will turn out much better instantly. Same works for barbers and real estate agents.