they're providing an entertainment that people are willing to pay them that much to see. Same with any musician or movie star. If you don't think its deserved then go spend or donate your money elsewhere.
If grounds are full, then people are willing to pay that much to be entertained by the product.
Sure, people can go and spend their money anywhere/anyhow they like. It's just a shame that fans, especially football fans, you are valued to some degree within your group/culture on how strongly you support your team and that means buying supporter gear and tickets to home and away games etc. - and clubs/corporations know this and can exploit you accordingly. Fans demand is very inelastic.
Man Utd's match day revenue was 109.1 million pounds and merchandising was 38.6 million pounds year ending June 2013. That's ~2.3 million people's cost of AIDS treatment for a year.
And even though people can go and spend what they like, I still think it is fair to have an opinion on the matter. And yes I'm a hypocrite, I spend money on frivolous entertainment that could be better spent elsewhere, I spend too much time engaging in sport when it could be spent better elsewhere - it's just easier to point out the extreme excesses of professional sport/most things really.