Just a little clarification for those who are still confused as to why there are contest restrictions preventing some countries from participating. In many countries there are gambling laws, and most often anything that is seen as a lottery or sweepstakes in which the contestant must pay in some detrimental way to enter, would fall under these lottery and sweepstakes regulations which the host of the contest must adhere to.

Some of these rules and restrictions may contain, but are not limited to:
-Registering your lottery/sweepstakes with the host country
-Pay a tax of fee to run the lottery/sweepstakes
-Offer an option of cash pay out equal to the prize of the contest
-Depositing security money equal to the value of the prize with the government
-Display odds of winnings
-Time restrictions on how long the contest can run before prizes must be given

I'm sure there are a lot more laws, but these are just some of the few that I can think of off the top of my head. Each countries have their own unique set of laws, and if SE is only able to hold contests in countries where they have offices (though I don't know if this is the case since I didn't read the entry restrictions yet), that's most likely due to those offices being able to get permission or a license from their host countries to hold the contest and is not because SE is biased against other countries. The legal fees and resources to get licenses in every country that SE sells their games would be quite large I'd imagine, so I can understand if they choose to reserve their resources for other things, like making games. Big corporations don't have unlimited money (unless you're a big bank with your government gauranteeing to bail you out of any bad financial deals you make).

A contest like this cosplay contest that requires a submission of your picture would probably be considered an entry fee of detriment because one can argue the time and money spent on materials for the costume is detrimental to the contestants. I hope this clear things up, though being the internet, I'm sure this rational explanation would be forgotten or lost a few pages into the thread and more complaints about the region restriction will continue to crop up in the future.