You have an a old coin, and someone finds a way to mint similar coins, made from the same metal, but they won't be as old, so they'll present lower value to a collector. One of these coins is given for free to everyone who holds the old coins, one for each old coin they're holding. Does that mean the value of the old coin will go down? No, because nothing has changed, it will retain its value, while the value of the new coin will depend on the market.
Your way of thinking implies that the makers of the new coin will take a hold of your old coin, chip a part of it and insert it into their newly minted coin to increase its value. Nothing like that will happen. Furthermore, you selling your new coin will have completely no impact on the old coin's value. These will be two separate markets.
Hmm..
So it means since traders are not actually buying BCC but getting it for FREE, there's virtually no pressure on king BTC.
But BCC value may even decline due to minor (or less) support and the intentions of BTC holders to sell BCC for added cash and even buy more BTC !!! Still sounds strange tho