Premades and leveling.

So with balance and everything.. I was just on a team with people level 21-28 against a team with a Diamond, two golds, a silver and an unranked. Preamade, sure, but how is it considered "balanced" to put them in a game against people who don't even have full armor/run setups? I'm irritating by it because it happens 2-3 times a night and they ALL BM AFTERWARDS. So how is this considered balanced?