It's only unfair because the majority of US citizens do not care enough to change it. :\
Also because of the electoral college system which only votes Democrat or Republican.
Democracy in general is pretty unfair because it takes away choices that could simply be left up to an individual and decides for them according to what at least 51% of the population wants.
The Republicans and Democrats have to meet the same standards for political privileges as third parties. There are poll requirements, petition requirements, etc that all parties most subscribe to in order to be featured on ballots, be in the televised debates, etc.
That's kinda like saying the system is unfair to minorities because they involve less people...which makes sense. If not many people care about a certain base, then it's pretty pointless to put too much stock in it for the entire country
Most 3rd party groups aren't trying to win elections; they're most often parties with one goal which want either of the two larger parties to adopt their policies.
A lot of third party candidates don't run to win but to bring their topics of interest into the public.