Because they are "not the same banderites". The modern use of the slogan managed to get heavily rehabilitated over years, also it originated considerably before conflicts (increased by polish fascists who effectively took over sanation government) led to massacres (which happened both ways, and similarly current polish government essentially does the same thing as Ukraine with Stepan Bandera, arguably longer - it started before they got into full power).

And well, we're next if Ukraine falls, or such is the feeling among considerable portion of polish people. Even pure self-interest makes it clear which way our support is going.