Quoth wikipedia:
The word professional traditionally means a person who has obtained a degree in a professional field. The term professional is used more generally to denote a white collar working person, or a person who performs commercially in a field typically reserved for hobbyists or amateurs.
In western nations, such as the United States, the term commonly describes highly educated, mostly salaried workers, who enjoy considerable work autonomy, economic security, a comfortable salary, and are commonly engaged in creative and intellectually challenging work.Less technically, it may also refer to a person having impressive competence in a particular activity.
Perhaps it's a bad choice of words - but what else should they call themselves?
It could be argued that in most other fields, the title 'engineer' is generally held by people with engineering degrees - this isn't the case Australian aviation, so I'd guess they feel they need to add something more to the plain old 'engineer' title to distinguish themselves.