A LOT of companies tend to keep positions listed on a constant basis even if they're not really hiring for that position. Stores will often list that they're hiring for cashiers but quite often they're not really hiring unless it's the start of school or for the holidays. They collect resumes and when a spot opens up or expansion is done, they have some early applicants to look over along with new ones. In other cases, they do it to make it look like they're doing better than they really are. Economy must be doing great if people are hiring ... right? Yeah ... positions listed doesn't always mean they're really hiring at all.
Blizzard's had those positions listed for years, though. I've been looking at it on and off for some time. It's nothing new. We do know Blizzard's expanding their teams, though, so this is a good thing.
And quite often people do jump between jobs. These days, people are lucky to stay 5 years in the same job before they jump ship to the next one. Between job security, the economy, salaries, and job satisfaction, lot of people jump from one place to another. So turn over rate of jobs isn't a shock, either.