I am sponsoring my parents for a visit to USA. The travel agency in Nepal told that each of them must take at least $500.00 from Nepal Rastra Bank and have a stamp in passport to travel otherwise they may not be allowed to board airplane Kathmandu Airport? Is that true? New law in effect?? Just wondering why they must bring at least $500 while I am sposoring all costs.
Has anybody heard about this or gone throug this? 

Yes that is true. Regardless of your bank balance, your parents must have that $500 stamp on their passports.