This surprised me too, but in fact, all ETH holders are also ETC holders and this is "free money" they can use to buy more ETH, and as such, generate demand for it, and sustain its price.

The total market cap of ETC being of the order of $150 000 000, and the daily volume of ETH trading being of the order of $ 8 000 000, even in the assumption that all of the ETC is slowly dumped in order to buy ETH, this means that the current price and volume could be sustained for 20 more days just by dumping ETC and nothing else.

I know this explanation doesn't fit with the earlier, much higher trading volumes of ETC and ETH, but there have been a lot of back-and-forth movements to exploit the volatility swings between both coins.