Selling a Banksy

If the art work is made in a public space, does the artist still own it? Or does the owner of the wall own the art work? This article discusses a situation where the owner of the pub transferred Banksy's piece onto a canvas and is now going to sell the piece in America.

Below are two links to the same story. The first one asks the questions that I have mentioned above: 



http://cornellsun.com/node/47027



http://www.guardian.co.uk/artanddesign/2011/apr/21/banksy-kissing-copppers-sold-america