It's a standard escape clause, especially in the United States of America, which ain't as united as you may have been led to believe - especially when it comes to laws applying in different states. Things like tax laws, driving rules and marriage laws vary to a fairly large degree from state to state in the US. It is common for packaging to say that the buyer of goods has no rights at all, except where that is prohibited.
So, just be carefull where you go to cash in those XP! Some places they may not be worth anything at all.
DWIM is Perl's answer to Gödel