There are no less than 3 views on the file.

1. Thumbnail.
2. The bigger view to the right of the explorer file pane.
3. The right click on the file and selecting Preview.
4. Right click and Open the file in Excel.

From memory item 1 does not require a password, item 2 may be true as well but 3 and 4 should require the password.

I think folk thought this was a bug. Call MSFT or post in their forums if you want to get it straight from the horse's mouth.

You could "make my files private" in the meantime.