Aviation is no different than any career in terms of the hiring of expat workers, it is a matter of supply and demand and there are plenty of pilots available to cover the demand, there aren't enough doctors right now for example and in the US (according to a friend of mine that works in that field) there have been more work visas issued to doctors from other countries than any other time in history. So the question is simple really, Can the demand be met? If the answer is "yes" then it is unlikely that you will see work visas associated with job offers in the aviation industry.