This is just speculation, but based on a conversation with my partner, there are two types of computer users. The first group makes use of hierarchical storage, consistent naming conventions, and other organizational tricks to give them a rough idea of where any file might be. The second group has never heard the term "file systems" and just stores everything with an arbitrary name in whatever location the originating application uses by default.
The first group would prefer to search the given directory, because the supplied context (of which folder to start the search in) drastically improves/speeds search results. The second group prefers to search the entire disk, because supplying that additional context is impossible - any file might appear anywhere.
The set intersection between the first group and "people who change their default settings" is much higher than it is with the second group. Consequently, the whole disk search is enabled by default.
Additionally, given the addition of an "All My Files" view in Finder (a feature which the first group would probably find baffling), Apple may also believe that the latter group outnumbers the former.
As search is fast enough nowadays with indexing, I'd rather have it search the whole disk every time than the directory I'm in just to realize I've put it someplace else.
Maybe we need a name for this behavior, I propose "idiot-driven-development".
