Indeed they are not tied to it but there market cap creates a valued base for the prices of all crypto currencies and mainly bitcoin. When money is flooded into the market then it creates huge demand against the purchases of different coin. Know that the coins are all limited in number thus this demand creates less supply situation all the time. With the difficulty of mining not much coins are generated every time and thus as whole there is large demand and less supply situation and thus giving the price to the coins. Thats how I understand the whole thing here.