FFS, it's not about the right's of the fatties it's about the rights of the 'normal' people. You pay for a seat, it's not unreasonable to expect to sit in it. People have been left with long term crush injuries after enduring a flight with a lump on top of them (though Gods knows why they didn't complain). Even in a more normal situation physically touching some big fat lump for an entire flight is pretty unpleasant (and I'm not having a go at overweight people here, most people are fairly fragant after 12 hours on a plane).
The funniest thing i've seen on flight was 3 fatties all sat at the bulkhead as they couldn't or wouldn't fit into normal rows. All was fine until feeding time, everyone else flipped the seat back tables down, being on the bulkhead their tables were the ones that fold out from the seat arms. Unfortunatly they were too fat and the tables wouldn't unfold over their food babies. In the end the fligh crew had to find some trays so they could balance their food on their stomachs, twas very funny.