The first thing is that Cameron did not award the MBE to his barber. He might be known for cutting the Prime Ministers hair but that is probably just a coincidence. He has also cut Madonna's hair, Jason Donovan and was the official hairdresser for the X Factor contestants plus Paul McCartney and loads of others. So you could say "Madonna's barber gets MBE"
The second thing is that he was awarded it for "services to hairdressing" which might seem a bit silly. However, he has set up his own, funded by him, apprentice scheme for young people taking up hairdressing and this is what he is renowned for. He was nominated by a hairdressing association for campaigning to have young hairdressers given a proper qualification.
Thirdly, the Prime Minister may nominate people for awards, which he didn't in this case, but he has no involvement in the awards themselves. An independent committee handles that.
Still, it makes a good story for the press "reporters and columnists" to get their hands on. Considering the fairly high awards given to people who are sometimes good at sports (How many Sirs, Dames, CBE and OBE are given to people for wining a gold medal?) such as the England cricket team (I wonder if they''ll be asked to give them back after this last test) probably quite good to see someone a bit out of the public being recognised.
Oh, and the Queen herself bestowed a warrant on her hairdresser and the Duchess of Cornwall's hairdresser got an MBE 3 or 4 years ago.
And any organisation that can award a peerage to the likes of John Prescott, Jeffrey Archer and Neil Kinnock has already plumbed the depths anyway.