My guess is that if they refuse your spouse during the interview, they believe that he/she is not a genuine spouse, and since he/she is currently in Canada, they dont want someone who's been determined to be a non-genuine spouse, to continue to remain in Canada while going through a potentially lengthy appeal process if there was one.
Like for example, how some refugee applicants can continue to remain in Canada for years while appealing their cases, I'm guessing CIC is trying to avoid the same situation here