There are people who think that “going through the motions” of Christian ritual, including wearing a cross, somehow make you a Christian.
I see your point too. I had the misfortune of living with a very abousive and very religious couple for several years. When they wear religious articles they are only fooling themselves and everyone else for that matter. They will be dealt with....by a higher power than Rowan Williams. It is not up to him to decide who believes enough, it is Gods.
No it is not up to Rowan Williams to decide who believes enough. But it is up to him, as a minister of the faith, to point out where he thinks people are going wrong. If someone wears a cross and it supports and upholds them in their faith, then his words are not for them. If they are using symbols as a substitute for faith, then his words are a challenge.
Incidentally, he has just announced he is stepping down as Archbishop of Canterbury in December.