Why do they do that?

I noticed a couple of weeks ago that Fox News kept on referring to Osama Bin Laden as Usama Bin Laden. They even write it that way. I did a little research because I had never heard him called Usama before. Sure enough, that is an alternative way of pronouncing the man's name. I am pretty sure they didn't always do this though.

I haven't seen Usama used anywhere else. And, I am confident that they used to refer to him as Osama. Why change now?