This is a hypothetical question, but I am curious. Feel free to ignore it, when it seems too theoretical for you.
Some people may know the situation that you implement a feature and in the end you have added like 10 lines and removed 100 and from the 10 there are possibly 5 that just were moved around and not really added.
Now suppose person A writes code and releases it with a strong copyleft, e.g., using the GPL. Person B contributes by this code and improves its efficiency by removing redundant (but not dead) code and in the end person B contributed a lot of deletions.
Afterward, person A wants to re-license the code with a weak copyleft, e.g., using the BSD license. To do so, he or she asks every contributor to relicense his/her code and removes any code from persons that cannot be reached or not willing to re-license.
Let's say person C is rewriting the missing pieces and licensing them under the BSD license, so there is no conflict with person A already knowing the removed code.
Now there is a problem: person B contributed a lot of deletions, but the deletions made a significant difference, so you would think that copyright should apply. At least for the diff (containing the content to be removed) it certainly does.
How does copyright apply to this case to the resulting code (without its commit history) and what can person A do with the GPLed deletions? You could say he cannot keep them, as then the project stays the same as after the work of person B. But should he re-add the deleted code?
Even when he does and person C removes it again, the result is the same and looks just like the code with the GPLed deletions, so it cannot be free of GPL work (i.e. the negative code).