Think of it this way: Heating wood makes it more brittle. The belly being somewhat brittle can handle that much better than the back of the bow. It stiffens the belly, adding more compression. The back can break much easier when brittle.
That said, do you remember just what Isi was doing and how he was doing it when he heat bent his bows? I've, also, read that heating a stone, wrapping very moist moss around the limb and applying the heat to the moss where you want the bend is one way the indians did it. But it didn't say which side of the limb they applied the heat. If the wood in the area that is being bent is pretty thick and the recurve will be static then possibly heating from the back could work as the wood at that point won't be bending. But it could be that they heated the belly side for a while and then bent the limb tip over a log, too.?.