Shmacky-McNuts: Im an american, I can be pretty darn ignorant and you cant stop me! ;)  
 My point is, GOG is not in the USA. They have no obligation to abide american laws, as Poland is not apart of america. 
  Yes, a company (or person) in Poland has no obligation to abide American laws, that's true. GOG does not have to abide by American labor laws or environmental laws or many other laws. But, once a company (or person) decides to do business in any state in the US, in order to do so legally they must follow the commercial laws of that state (regarding advertising, guarantees, and yes... sales taxes. It's a "choice" by anyone in another company to make. Do I want to do business in Mass. for example? If no, I don't have to follow any Mass. law. If yes, then the only way that market opens up to me is by obeying their laws. The same, btw, is true when American companies do business in other countries. They must follow their laws in order to do business there.  
 A far more interesting question (to me at least) is how VAT works in all of this. Because I live in a state that has a sales tax, but I have never, ever been charged a sales tax here. Not once (just made a purchase yesterday for the record). Yet others do every time. My speculation (and that's all it is at this point is speculation) is that because the VAT in all European countries is already included in the price (which IMO is how it should be here too but 50 different states and taxes makes that somewhat difficult) so maybe they're only charging the tax in states where their sales tax rate is above the VAT tax? Does that sound like a reasonable speculation or a weird one? I dunno, it's the only thing I've been able to come with as to why some pay them and I don't when I do live in a state with a sales tax (mine is 5%)   
PixelBoy: Have you read the legal stuff? 
 GOG has some special arrangements for American customers, so presumably they have some kind of office in the USA, at least some legal service.  
 "For users in the USA only:  
 19.3 You and we agree that your use of GOG services and GOG content, and this Agreement, will be deemed to be entered into in Los Angeles, California and governed by and interpreted according to the laws of the State of California, USA (and, if applicable, US Federal law). Any legal claim by you against GOG will be made exclusively in any state or federal court located in Los Angeles, California, which will have subject matter jurisdiction regarding the dispute between you and us and therefore we both consent to the exclusive jurisdiction of those courts. In any legal claim under this Agreement, the side which wins will be entitled to its legal fees and expenses." 
  I don't think that pertains to sales tax laws (could be wrong). It's possible (likely?) that it's only about resolving disputes (such as violations of a refund policy or EULA or some consumer protection laws). It's usually the case that tax laws are specific to the state the transaction takes place in. In other words, if you live in Oklahoma, you may have to sue in California courts (or a federal court located in California) for redress but you won't have to pay California sales tax.  
 But I'm no lawyer and I didn't stay at a Holiday Inn Express last night either, so what do I know? lol