There is no friendzone.
From experience of being on both sides, it's just somebody being too gutless to say they no longer want contact with you after either a relationship, a fling, a one night stand or you being far too keen with your emotions/intentions.
I've never wanted to be friends with a girl I was no longer sexually interested in, I already have friends. I don't need the awkwardness.
It works both ways, there's times where I've wanted something that clearly the other party didn't, which lead to the 'just friends' line. Don't think I've spoken to either girl since. First one was around four years ago
The basis of friendship is that its based on an equilibrium. There shouldn't be a dominant party, one that controls the relationship.
By someone already making the decision to 'just be friends' it is clear that there is a controlling force and such a balance could never create a give and take relationship like the one shared between friends.