Wouldn't posing as Doctors, lawyers and CPAs give them a high profile? Isn't that what they would want to avoid?
Your implication is quite sensible--that being a professional in the US is a time-consuming and expensive cover to use.
However, if you read about the backgrounds of the terrorists we have caught and know about, they are frequently educated in Western institutions, and are the upper rings of middle-class society. One very highly-placed terrorist was a physician.
Despite the logic of your comment, I think that Middle-eastern professionals ought to be scrutinized as carefully as anyone else.