The world of IT is full of these "contractor" behaviours. I've worked in it in the UK, but not Germany.
In the UK, if you can prove that in the contract, it is not for you to do the work, but for you to provide a skilled person to do the work, i.e. that you can substitute someone else who has the required qualifications and skill, and if you can show intent to "sell" your service to some one else, then you can get away with it.
I do not think this applies to Aviation.
So, as and when France, Germany and others catch up with these dodgy employers, and when the Brit politicians focus HMRC on the issue, then something might change...................Some time away, yet.