The reason for region restrictions is this:

They have different price policies depending on the region!

This is one of the best things any global company could do. It is an example to be followed.

A game that costs five dollars in the US may cost the equivalent of 2 dollars in another country. This is due to different economies. A simple currency conversion would make games more expensive in developing countries.

Once people discovered that, they were taking advantage by using proxies or asking friends to buy games for them then sending the keys ...

They decided to do region block because this.

This is awesome, giving an opportunity for people to pay the right amount for a game, based on their local economy.