Have they never heard of Herr OŽLeary then ?
FR has no staff based at SXF. No FR staff in Germany is organised in a union. It's of course a different story why this is so. I don't think I ever had German staff in a FR aircraft, though there must be some at the HHN, NRN and BRE bases, but even there hardly any German nationals. Quite typical for Germans to leave low-paid and low-profile jobs to EU or Turkish immigrants.
EZY staff at their SXF base is organised in the powerful service union VERDI (millions of members) . SXF is EZY's only remaining base in Germany (probably for exactly that reason). The union has limited interest in the fate of Eastern European workers (as you can read in recent interviews about the 1st May job market opening).