Why does the European Union insist on 'free movement'?

In his Bloomberg News column lamenting British voters’ decision to leave the European Union and speculating on whether it could be reversed, British-born Clive Crook also, and more interestingly, challenged German Chancellor Angela Merkel’s insistence that the “four freedoms” proclaimed by the European Union, including freedom of movement from country to county, are indivisible. “The price of access to the single market means freedom of movement,” Merkel said.

Nonsense, Crook replies. “This position makes no substantive sense. The indivisibility of the four freedoms is a mere assertion — a preference, not a requirement of fairness or logic.”