Complement vs Opposite

From Zanecorpwiki

We often use the term "opposite" when "compliment" would be better. Pretty common... Chris Anderson in "Free: The Future of a Radical Price" (TODO: link), he says, "Freemium is the opposite of the free sample. Instead of giving away 5% to sell 95%, you give away 95% to sell 5%." The idea of compliment, both the general and geometric, is very close to the idea. "Opposite" however implies opposition, which isn't really the case. They're different, but opposite?

Yet, we seem to really love the word "opposite".