Considering they're responsible for over a third of the bandwidth used in the U.S., it makes sense that they would take this stance. Less competition means more chances for Comcast to make demands.
Regardless, we need a high-profile company like Netflix to come out and say this. If they can paint this as a terrible thing to their own customers, hopefully they can sway the attitude of Americans who might otherwise not have an opinion on this merger. It's bad enough that Comcast and Time Warner are viewed as two of the worst companies in the U.S. (and rightly so), but seeing them merging into one would be disastrous. We need to be breaking these companies apart, not allowing them to congeal into one supercorp to rule them all.