Our general policy is to pretend the registry does not exist
That's why AIW can run off a jump drive if you really wanted it to, etc.
I'm not sure if the BitRock installer builder we use for these has an option for registry queries to populate the destination field. Anyway, I think the general idea behind the standalone installers is that they're probably being used by someone with a standalone copy of the game (in which case the auto-selected path there would generally be correct), and that someone wanting to install an expansion for a steam copy would be getting it through steam (and thus not need the installer at all). There are always exceptions, of course