Is yoga un-Christian?

I came across two stories related to this today, this and this I don’t get it. Aren’t these the types of people who were asking for a moment of silence in classrooms? They now have their moments of silence, in spades.

Yoga is a form of meditation. Meditation does not play the same role in Christianity that it does in Hinduism and Buddhism, but it certainly is a part of Christianity. Quaker workship services are one long meditation. Many Christian groups organize retreats featuring periods of silence. I have seen Christian interpretations of meditation as a means of surrendering to Jesus.

The only thing that I find disturbing is that the military has found a way of turning a spiritual practice into a way of creating better killers. If the military can find a way of co-opting yoga, Christians should surely be able to do so as well.

I am putting this in the general section to avoid snide comments. What I want is a simple explanation for what is upsetting these people.