It is not that they don't believe it; rather that it is only part of the story - again as merely the latest example of Arab paranoia about the U.S., Israel etc. . . . Witness the amazing Mossad shark.
Or are you going to try to tell us that U.S. support for the Egyptian Government prompts statements such as "the goal of the US is to weaken and divide Islam."
What part of "partly" is it you don't understand?