I'm not trying to make excuses here, but maybe the 'bad-niks' saw the sweaters on the trees and got to thinking the moose might eat them and get a belly ache. Maybe they thought kids had put them there. I mean, it took someone a lot of effort to go and work at cutting them all off.  I don't know, it's a conundrum.  Next time try putting up a little sign explaining the purpose and that the yarn and its dye is 100% digestible. 
Hopefully no one will remove them to snack on!
Anyway, many thanks for your well meant efforts to bring a cheery note to the icy forest in dead of winter.