Except most big companies stream their conventions for free because these things are marketing tools. Thats why E3 or Apple announcements are streamed for free, because there’s more value in the long run from the marketing power of these things than their is in their value as a ticket. Blizzard are just greedy. And I'm not saying that because I hate Blizzard, I'm saying that because when you look at the way they behave they come across like they care more about making a penny now than a dollar tomorrow.
Edit: I now feel silly for not checking how old this thread was.