Of course, I understand where the Polish government is coming from. Like Germany it already bans Nazi paraphernalia. Interestingly, neither country needs to ban Mein Kampf, because the German Land of Bavaria owns the copyright and has always prevented republication. However in 2015, 70 years after the death of the book's author, the copyright will expire and new laws will be needed if it is to continue to be suppressed.
The Polish people have lived under both Nazism and Communism. They had a hard time of it under both. It is scarcely surprising that some should see it as inconsistent to ban the propaganda of one set of totalitarians, but not the other. We libertarians, of course, would ban neither. I cannot imagine why anyone would wish to sport a swastika, but I certainly would find it no more offensive than "CCCP," the hammer and sickle or Che Guevara on a T-shirt. If people want to reveal themselves as supporters of evil ideas, then that is good for public safety. It is the unseen supporters who are dangerous. Far better therefore never to set out on the treacherous path of banning.
If Adolf Hitler is too evil to feature on a T-shirt, then why not ban Che Guevara too? Or Marx, Lenin and Stalin? The problem is - as always - not where to start but where to stop. If Trotsky, then why not - for example - Mohammed? If Mein Kampf, then why not The Koran? Far better never to take that road, than to face such choices. Far better to have all ideas aired openly, the better to defend ourselves against them.