> Just a guess but since apps can fail to free memory correctly
That's not relevant here; from the perspective of the kernel pages are either assigned to a process, or they're not. If an application fails to free memory correctly, that only means it'll keep having pages assigned to it that it no longer uses, but eventually those pages will always be released (by the kernel upon termination of the process, in the worst case).
That is the worst case if the process had leaked that part of the heap, but it is an optimal case on process exit. On OS with any kind of process isolation walking over most of the heap before exiting as to "correctly free it" is pure waste of the CPU cycles and in worst case even IO bandwidth (when it causes parts of the heap to be paged in).
Pages can be completely avoided to be paged in if the intention is to just zero them. The kernel could either just "forget" them, or use copy-on-write with a properly zeroed out page as a base.
The point is that you do not want to do any kind of heap cleanup before exit. The intention isn't to zero the pages, but to outright discard all of them (which is going to be done by the kernel anyway).
That's not relevant here; from the perspective of the kernel pages are either assigned to a process, or they're not. If an application fails to free memory correctly, that only means it'll keep having pages assigned to it that it no longer uses, but eventually those pages will always be released (by the kernel upon termination of the process, in the worst case).