A visa is basically you asking the country you want to enter if you can come in. If you get one you can go there for whatever period of time it allows you.
The United States (and other countries do this too) is saying to the young 20 something players with very little at home (money or property) that they are a flight risk and will just stay in the US instead of return when their visa is up. So you have people from the poorer countries in the SEA and Easter Europe getting their visas denied by the government so they can't go to TI5.
Valve can do nothing about this because it's a company and not part of the government. But that doesn't stop people from begging and pleading on reddit.
damn. That is fucked. Gaben gonna have to grease some palms to get them in. Crazy shit, thanks for explaining.