It basically ruffles some of their feathers because they have such a big and justifiable idea of what Man United is. They're not used to being rejected so I guess some fans want to know that he regrets his decision. When they realise that he doesn't, they then try and go 'yeah well we won loads of trophies'...some of them can't comprehend someone wanting to play for their home town club without winning anything in the same way that some of them can't comprehend supporting their home town club without winning anything.