It is against the EU regulations to differenciate between member states in such manner, but Apple justifies themselves saying that due to intelectual property laws in Poland it is impossible for them to sell iTunes tracks to Polish customers. Obviously, they are not trying hard enough. The only think you can do is to stock up on iTunes gift vouchers when you are in an Apple-category-one coutry.
Alternatively you can just download music for free. I would like to have legal copies of my music but if the record companies are too stupid to sell it to me....