Well there ARE girls a person CAN just be friends with. I suppose there is a technical rule that in order to exist in the 'friend zone', you have to have 'more than friend' feelings while being a friend. Whether or not you are stuck depends on various factors.
Being PLACED in the friend zone, ie the girl secretely knows you have feelings for her but has decided you will stay her friend, means you are stuck there. It means that somewhere along the way you failed one of the more arbitrary marks. However, you DO serve a utilitarian function, like a can-opener, in that you are a relatively neutral party that will comfort her when times are rough, and aide her while she rallies against failed relationships.
Then there's the mutual friend zone where two parties both, sometimes verbally, agree that staying friends in the best. This is usually the best way because it doesnt = someone manipulating an emo sap that cant let go.
I mean I guess I can agree its complicated, but the ZONE itself is pretty clearly mapped.