The police have very little discretion anymore. If they'd refused to arrest the guy, then they'd have opened themselves up to all sorts of bother. The CPS wont prosecute.
As both a journalist for The Sun and a former resident of the Isle of Wight who has seen this chap play quite recently, I like this news story very much!
While I personally hold that anyone singing that stupid song should be taken out and shot I feel the arrest on racism charges is silly and a besmirchment on the individual's character.