Perhaps I should have said "For me, football should be a sport first and foremost". Do you think football should be a business first and foremost?
The difference with tennis and golf, to use your examples, is that earning money doesn't make anywhere near as much of a difference to your chances of winning as it does in football. Federer and Woods have been very successful in their respective sports not because their prize money has allowed them to get better equipment and coaching, but just because they are naturally amazing. United, on the other hand, have been very successful in football in large part because their prize money allows them to buy better players than everyone else. (Of course Fergie has played a big part in their success, especially initially.)
As I said before, you evidently think of fairness mostly in terms of being allowed to keep whatever has been earned legitimately. Personally, I place more importance on equality , ESPECIALLY in sport. This fundamental difference is why left-wingers and right-wingers tend to argue forever when it comes to certain things IMO; they just prioritse a different set of values over another. Which is fair enough. But when it comes to football, I find it really difficult to understand how anyone can feel okay with the level of equality that currently exists.
You say United's earnings are legitimate, but they're only legitimate because whoever made up the current rules decided they are. In the past, clubs weren't paid money by companies who wanted to sponsor them. They weren't paid money based on how often they appeared on television. They shared their gate receipts with each other. They didn't get disproportionate prize money for finishing in the top 4. If the FA made a new rule which said that teams who finished in the top 4 would be rewarded by being allowed to field 12 players next year instead of 11, would you be okay with it? I know it's a silly example, but I'm just trying to show that playing by the rules isn't necessarily fair if the rules themselves aren't.