It's more of a salient point; why is the Utd fan not addressing the fact his own club acts the same way as the ones he's criticising, and more to the point why would he use it in an effort to portray those institutions he has a bias against, in order to influence others to join him in his condemnation of those clubs, whilst dodging criticism of his own?
If your arguement is being used on those premises, only to find it is weak to begin with, don't defend using it. That is precisely what "whataboutism", when used properly, probes to expose. The weakness of an argument based upon someone's bias and hypocrisy. It can easily be argued that the act of spending money on players to gain an advantage is a heinous one, and that in this instance the Utd fan is calling that ALL clubs who do it, including his own, should be be brought into question.But he's not, he's using it to bring others onto his side of the argument, ignoring the actions of his own club and getting riled when this hypocrisy is brought into light.
Rarely however, in these shouts of "whataboutism", is that the case.