Student: read previous posts in the thread and you'll get all the explanation you want.
Autopilots generally control pitch using elevator trim, and can "disguise" a serious out-of-balance situation that's developing due to icing by just dialing the trim slowly out to the stops...
Which can lead to a very nasty surprise for the pilot when the AP is turned off for landing - in the case of a long-lever-arm plane like the Q400 perhaps a ton of trim force trying to point the nose or tail at the ground (depending on where the ice is)
Kind of like your instructor tossing overboard a box of cement blocks tied to the tail or nose at 1600' - and then saying "Your airplane!"
Flying manually, the pilot will notice "Man, this thing is getting really nose/tail-heavy" through his/her fingertips long before the plane becomes fully uncontrollable, and do something about the situation more creative than the AP's trim-only solution.
I should add in light of the following posts that:
the NTSB makes RECOMMENDATIONS
the FAA make RULES and LAW
the airline sets and enforces company POLICIES & PROCEDURES
the manufacturer recommends procedures
It is probably a very good idea to fly manually in icing - but the law doesn't always require it. And even the NTSB seems to be backing away from putting too much blame on using th AP - yet. As would I.