Mmm. Please, note that we did this intentionally because, technically, it must work in that way to avoid problems with stock calculations.
You can have backorders on individual products but not if these are part of the BOM tree.
Let me describe an example:
If you have BOM stock control enabled, all the final products have their stock calculated by their BOMs, and multiple products can share the same BOMs (for example, product A and product B). So if you sell product A and its BOMs go to minus, how would we calculate the stock for their parent to reflect the backordered product?
And how we would calculate at that time the stock for product B that is sharing the BOMs with negative stock in this case and is not really backordered?