So I have a few theories here having to do with Lotso, and the climax of Toy Story 3, and why he didn't press the stop button... thing. Well here we go...
- He didn't think they could escape even if he did press the button. See, notice he's like twice their size and he had quite a bit of trouble reaching the ladder to get out. I mean look at Lotso compared to the other toys. He's huge!
- He thought if he pressed it, they would still end up in the incinerator as they were literally standing on the edge of the conveyer belt. After all, if the conveyer belt had stopped suddenly, the toys might have been sent flying into the incinerator due to inertia. (Or they could've just lost their balance.)
- And one last thing, note the Little Green Men would have been way too small to reach the controls of the claw. So how exactly did they get up there? Lotso was giving them a boost! They all got up on the seat and Lotso helped them reach the controls by holding them in his hands and lifting them to the controls. So when he left the incinerator, he was actually going for help!