Interesting.. I have wondered the same. I can see smaller brands making inspired pieces but when its 95% identical minus the label I think its treading on thin ice. Skulls, the speedy shape, ugg look a like etc are pushing it a bit far. Just look at Blue Nile they have many pieces identical to Tiffany's, it offends me. lol
Bearpaw and Emu make well-made boots that are similar in shape/size to Ugg, but are a fraction of the price. They still use nice materials and their boots get the job done (at a fraction of the price). If your goal is to keep your feet warm and you don't care about appearance, it makes more sense to purchase the less-expensive item.
I think that it's about liking the way an item looks as opposed to who made it or trying to pass it off as something else. I bought a pair of Bebe heels that I looooove (I actually have two pairs of them in the same color). While wearing them shopping at Off 5th, the salesgirl thought they were LAMB because they were identical to the order they'd just had come in- she was right!! It didn't mean I stopped wearing them once I found out they were a look-alike; I bought them because I liked them.
IMO, it's not okay to buy something and pass it off as something else, whether it be a fake or a look-alike.