Firstly I would like to say that I am one of those who has and will fly outside of the Airline. In some and most of our cases we do have written permission to fly and train outside of our airline, as long as we stay within company flight and duty limitations.
There are always three sides to a good argument.
From my point of view and what I have seen there is a very limited amount of Airline pilot’s that actually do freelance work. Hence, feel the witch hunt seems more like a personal vendetta.
On the other side of the coin it is not possible to keep all the proficiency ratings current and fly without a lot of dedication. In other words the income versus effort is very limiting; hence I very much doubt they do it for the money.
It is a generalisation but many guys who fly freelance are dedicated to general aviation and do the work with passion, passing on information and knowledge. Usually there is a lot more background to each person, than pure financial greed. As QNVS said you may learn a lot from them.
Professionalism is NOT resorting to ‘gorilla tactics’ to destroy some-one else’ input in General Aviation, but rather improving your own.
This industry is small, and usually the same guys who are involved in GA are the ones the help advance the careers of the people they meet. But it can work in reverse.
So in summary, concentrate on your own career and not other peoples. I am sure they are not stealing that much bread from your table. You may well find yourself one day wanting to put/get back to GA.