After setting a value for the ContetTypeId for a view (custom or existing - e.g. AllItems) go the Actions-> Edit in Datasheet.
Instead of getting the datasheet view you will be redirected to the Root Folder of the document library.
Solution:
Actually this is more a workaround than a solution...
Create another view with the same fields as the one that one you want to use in that specific folder (or any other content type inherited from Folder) but base this new view on datasheet view instead of standard view.
Instead of doing Actions -> Edit in datasheet, the users should just change the current view to the one based on the data sheet.
