Because by igniting the fuel, which logically nobody should have been able to see, they went from a slight chance of surviving returning from orbit to zero chance of surviving their orbit.Secondly, HOW WAS IGNITING THE FUEL NOT LOGICAL. It literally gave the Enterprise the signal needed to rescue them. It was surpassingly logical, it was T'Pau-tier logic. He should have been given the Vulcan Medal of Supreme Logic for that, while Kirk should have been handed the Unbelievable Dunce Clown Award.
Even when he did it Scott and McCoy knew he was making a nearly impossible gamble. One that if it hadn't worked out there would have been no return from.
One could argue that their chances of surviving on the surface were slim. But really, there had to be places on the ENTIRE PLANET where the natives were not. And they would have been landing someplace other than were they had left from. (Where they had not pissed off the natives.)
But we also know they would have been stuck there. OTOH knowing our Brave and Heroic Captain, once he dropped off the medical supplies he would have returned and continued the search.
I'm not saying everything would have worked out OK. It probably wouldn't have. But once Spock ignited the fuel they were dead.