Many people respond to these policies with "Yes! Make the fatties pay for the extra space they take up."
I've said this before and I'll say it again:
Airlines are doing this to deflect attention away from the fact that the amount of space they offer is too small for most people. If they provided adequate space, people wouldn't feel so defensive of the amount of space they get and wouldn't be all "OMG fatties in my spaaaaaaace!"
Once people accept this, airlines are going to respond by making their seats even smaller, thus forcing even more people to buy two seats.
When I am traveling alone, I buy two seats. If the flight is full, the flight attendants usually try to lay claim to my second seat to put a passenger in, so I have to tell them I paid for it. Thanks to years in the fat activist movement I'm not embarrassed to do so. In fact I am even looking forward to the day I have to say "I bought that seat because as should be obvious to you I am the size of two ordinary people. I work hard to maintain my enormity, and if you don't leave me and my second seat alone I will have to eat you."
Kate Harding's post on this has some very thoughtful comments: