From her pov, she's actually had a positive reinforcement that bad things won't happen.
Remember all the fuss about 500lbs? She skirted it for months, got to within 0.000001 lbs of it then frantically ate lettuce telling us she'd never eat badly again, she was converted, "apples are like candy" as her eyeballs swivelled in terror then, two days and a big dump later she was "safely" back down in the high 400s and ate to within a micron of 500lbs over and over again. She was frantic about it.
Then the inevitable happened, she absorbed it and admitted it and.....nothing. She'd built this entirely arbitrary number into a big deal whilst ignoring very real big deals, and all that happened was the number on the scale changed. Since then, she's gone balls-to-the wall on her personal pie quest. 600lbs doesn't seem to be the same rubicon - or not yet, though that might change as she gets ever nearer - so, to the crackpot pile of sludge she has in place of a brain, she's somehow beaten the danger. Even though it wasn't a real danger, she still gets that positive reinforcement and it skews her already distorted perception. Sleep apnoea, layyg falling off, veins getting clogged by a doughnut; it is, as you say, a temporary reeeeee but then it passes. Besides, she only eats doughnuts with holes in so she goooood booboo. Headpop.
But then that James K bloke rationalized a heart attack into gaining 100lbs, bawling OWURGH MUH LAYURGS! all the way. Must be something in the water in 'tucky.