All wheel drive/tires

I have a 2005 Subaru Forester 2.5, 46,500 miles, that I bought used on 12/27/06 with 36,000 miles on it. I am being told by the dealer service dept. That the left front strut is bent and the two front tires are bald on the inside edges. They are saying in addition to replacing the strut and an alignment, that I need to buy 4 new tires (all 4 tires were new when I bought the car 10 mos. Ago), because the tread differential between new front tires and the old back tires will adversely affect the all wheel drive portion of the transmission. Can this possibly be true? Does this mean every time I have a tire issue I will have to replace all four? This sounds bogus to me. Please help.