yea, I agree that is just plain dumb. Thank god we didn't get that tranny-AT setup !

A self downshifting trans when slowing allows you to return to speed without lugging, it downshifting for its protection. That function is a good thing.

Bringing this back from the dead - so in theory then, even if I'm accelerating in 6th gear at 40mph slowly to 75mph (as in traffic), no harm is done to the engine?

I notice the RPMs jump a bit higher if the car is going up a hill in manual mode, one time I was silly and forgot to downshift when going up a hill and was losing speed in 6th gear with the pedal depressed all the way. Hope that isn't bad...

I'm surprised no one actually answered this. The torque converter in your trains isn't a perfect science when driving, in comparison to a manual where direct engagement is governed by your foot. You're not hurting the engine, but the computer should protect itself, by later downshifting if it falls too low in the RPM's and begins to lug.

I use 6th on MT since 40mph in normal traffic where i don't need fast acceleration and for economy purposes, just to fit with rest of traffic flow, almost all the time. Not seen any ill effects of that. Yes, below 40mph(64kmh), i notice that engine sounds different, like below normal range of working, hence below i switch to lower gears. Or in cases where i need that extra acceleration for quick takeover/merge in and alike. I don't know if this will help you, as too lazy to check if gearing is same on MT vs AT.
Still, i guess that for cases where something may actually hurt AT gearbox or engine, it's management will override manual gear switches you do.