Raise your hand if you are a libertarian. Now raise your hand if you think Fidel Castro is a communist. I see exactly the same number of hands in the air.
Raise your hand if you are a libertarian. Now raise your hand if you think Gary Johnson is a libertarian. I don't see the same number of hands in the air.
Why do we think Castro is a communist? Surely not because his policy views are identical to some standard communist manifesto, say the works of Marx and Engels. For instance, Castro favors allowing some private business activity. Castro does not favor having the state wither away. Rather we view him as a communist for two reasons:
1. He has lots of communist attributes, with some exceptions.
2. We hate communism.
When insulting someone with the term 'communist', we just need to be close. (I'd guess a few people have called Hillary Clinton a communist, but most think that's going too far.)
Why do some of us libertarians (not me) deny that Johnson is a libertarian? Because these two facts hold true:
1. Gary Johnson holds many libertarian views, with some exceptions.
2. We libertarians like libertarianism.
Thus we hold our own kind to a higher standard. You won't find many conservative Christian fundamentalists who believe that radical Shiite terrorists are not "really Muslims", just because they are not Sunnis, but you'd probably find quite a few who think Unitarians or Mormons are not "really Christian."
I don't like this asymmetry. We should be comfortable with fuzzy definitions because they are useful. If we insist on very strict definitions, then almost no one will fit precisely the definition, out in the real world. It will be hard to develop a language to discuss political science, sociology, and related fields. Fuzzy definitions are useful. But so are adjectives, such as "moderate" libertarian, or "pragmatic" libertarian. It matters not whether you think Johnson "deserves" this or that label, all that matters is if the label helps to convey information about what Johnson is like. The term 'libertarian' is not a title to be awarded, like an Olympic gold medal; it's a tool to be used in conversation.