Personal tax is just that. Personal. The schemes by RYR and others only place the responsibility on the contractor. It is up to the contractor to pay the required contributions. Of course this means if the contractor paid the proper rates they would have less take home pay so many (not all) decide to be selective on how they declare what they earn. Citing expenses against income, dividend payments instead of income tax etc.etc.etc. Been going on for years, even in the UK.
If they prove they have paid everything owed then there will be no come back. If they've been evading tax then they deserve everything they get. I pay mine(UK PAYE) and am sick of subsidising those who think they can get away with it. Now where the German, and other countries, need to target their efforts is in the fact that RYR are not allowing fully employed pilots and thereby avoiding the employers responsibility for their proportion of tax. If the UK that is aboout 11%, so no wonder they try it on.