From the FAQ's:
Sweepstakes: Anyone can play in the sweepstakes, but only players in the following countries are eligible to win prizes: United States, Germany, Canada, United Kingdom, Japan, Ireland, Netherlands, Norway, and South Korea. Please note that some restrictions apply and not all of these countries are eligible for all sweepstakes tournaments. See individual tournament rules for details.
Skill Contests (Leaderboard): Players in the following countries are eligible to play skill contest tournaments with prizes: United States, Germany, Canada, United Kingdom, Australia, Norway, Ireland, Netherlands, Spain, South Korea, and Japan. Please note that some restrictions apply and not all of these countries are eligible for all tournaments. See individual tournament rules for details.
Skill Gaming: Ready-Go Tournaments and Match Play Challenges fall under Skill Gaming, which certain countries and US states prohibit.
The U.S. states that do not allow skill-based gaming are: Arizona, Arkansas, Connecticut, Delaware, Florida, Iowa, Illinois, Louisiana, Maryland, Montana, South Carolina, South Dakota, Tennessee, and Vermont
The countries that do not allow skill-based gaming are: Brazil, Argentina, the People's Republic of China, South Korea, Japan.