They're accusing him of doing it for publicity and personal gain. They seem unable to grasp the idea that for religious people, faith is not something that's taken out of the closet on Sunday and then put back in for the rest of the week. It's something that should inform and shape daily life. Because they are so empty spiritually, atheists probably can't understand the concept of being aware of God in your everyday life and being thankful for the blessings we have.
Lacking this spiritual foundation, American Atheists naturally assume that Tebow's displays of devotion are self-serving. How cynical and sad. I almost feel sorry for this group of in-your-face atheists because they will never know the true peace that comes from a close relationship with God.