I think its because Aden was sooooooooooooooooooooo boring, he didnt warrant much more than a cheer - it takes more effort to boo :D

Yes, Aden was not the most exciting housemate but as he's so immature, I think it was a kind thing to not boo. He's too fragile for boos. With that said, it hasn't been right that the girls have been booed in their evictions. They haven't been that bad. And I think that Tashie is another fragile soul.