iOS has always used case-sensitive HFS, while macs usually need case-insensitive HFS. Since it looks like APFS will drop support for the case-insensitive file system mode, it makes sense to do the forced upgrade on the system that already uses a case-sensitive file system (= iOS).

Plus, it's more straightforward recovering from a broken FS upgrade on iOS since many customers will have iCloud backups enabled, or at least their data stored on iCloud drive, while a mac filled to the brim with all sorts of strange applications and files spread all over the filesystem, likely never been backed up properly, is a tougher call.