The artist painted them on the street to spread art to the masses, not to be ripped out of the wall and sold to highest paying bidder at auction. Sure, if it is your property, I guess you can do whatever you like with it, regardless of whether or not it violates the spirit of the piece. The success of auctions like these proliferates this type of behaviour, not only from property owners, but anyone looking to make a quick buck. For example, there is a guy right now in NYC going around ripping down Invaders and selling them on Craigslist.
Why exactly would someone see this as a positive, short of having a large Banksy or other high profile artist's work on the side of a building they own?