Football is a privately run entertainment business. It is not funded by the taxpayer, so we cannot really complain about its wage structure. Rooney's services are valued at £15.6million. Presumably the people paying that must calculate his worth in terms of income he will generate, and have concluded he is worth it.
Manchester United earned £363 million last year, so they can easily afford £15.6million.
Premiership Clubs combined genered £2.5billion annually (2010).
The Premier League TV rights earn £1.7billion (£1billion for uk broadcast rights).
There is a lot of money in this business, and valued players and managers are handsomely paid.