You may be making some assumptions that might not be correct. In this video of a P-51 making a belly landing, it appears to me that that canopy has been moved back, i.e. to the "open" position before the plane makes the belly landing. Perhaps that was the standard procedure.
https://www.youtube.com/watch?v=ze2nEipDLR0
The second assumption is that the seat and shoulder straps were still closed (which I believe is the correct procedure for a belly or water landing). But what if the pilot first planned to bail out and then changed his mind, e.g. he was too low. I doubt he would re-connect the seat and shoulder belts if he had already released them. There probably was no time to do something like that. So I think it possible that in the examples given the canopy was retracted ( if not already ejected) and the belts may have been released. When the plane flipped over the pilot could have been thrown clear and then suffered a broken neck or massive internal injuries when he hits the ground.