The people who make pre-made groups are not looking for a fair fight, they are looking for easy wins. If pre-made groups only faced other pre-made groups, those people would complain and say blizzard is ruining the ‘fun’ of the game. Not taking into account the fact they are ruining the fun for others by doing it.
The proof is the fact there is already a system in place for pre-made battleground groups (though currently not usable until season 1) and that is Rated battlegrounds. However these pre-made group members generally wont take part in them because they consider them too sweaty, in other words… Too much of a challenge.
Premade groups exist to give casual but geared players who are not confident enough to do rated battlegrounds a place to stomp under geared enemy teams in order to have the illusion that they are actually good at PvP.
(P.S. - This post isn’t directed towards the people just queueing together as friends, trying to gear together and have fun. This post is directed to the baddies creating fully PvP geared 5 man teams to have a clear unfair advantage)