Aliens are allowed to live among humans because some of them have proven to be able to adapt completely to the ways of life of people on Earth. They want to study humans at a much closer level and this is the main reason why we are always letting them become residents of Earth for as long as they like. The Order has a lot of trading going on with plenty of alien species and some of the things we offer are quite similar to student exchange programs here on Earth.
We advise aliens to learn how to properly behave before we allow them to even try out a human suit in order to blend in. Once we learn that they are suitable for the job we allow them to freely move around, but their suits are all tracked and they are never supposed to roam this planet without them.