If I buy software from US company in US on-line shop – why do I have to pay VAT? It’s like exporting goods from US to Poland so why VAT is involved? Does it have sth to do with a fact that I don’t pay customs duty for software purchased that way?

EDIT:

Ok, here is the answer:
"The European Union has passed a law requiring non-EU companies to collect Value Added Tax (VAT) on sales of electronically delivered merchandise and services to customers living in the European Union. This law went into effect July 1st, 2003. This does not apply to items that are physically shipped. The VAT is calculated based on the country in which the customer is located."
It's like free money for any EU government- they take no cost & risk involved in software production (it's entirely on US side) – but they take 22% of the retail price…

On a purchase form I could theoretically enter fictional data (I own unpersonalized VISA) regarding country but I wonder (if transaction goes fine) – what consequences could occur?

It depends on the shop. When I buy from Telltale I don't have to pay VAT, but other places do charge (D2D did). I think the places that do simply have an agreement.

Most likely your actual country of origin will be verified via your public IP address, not the information you enter at point of sale, so unless you are also using a "spoofed" IP, you will probably still have to pay VAT.