It seems weird to intentionally limit access to the Diablo IV beta. I would think Blizzard would be eager to take advantage of player interest to actually stress-test the game's ability to withstand a rush of new players when it launches.
Surely you realize that testing the queue system is a part of this? In my experience, queue times are lower across the board than compared to last weekend (took only a few seconds for me right now, for example, in the middle of Saturday afternoon), which is not what I expected considering there are definitely tons more people playing. There's a bit more in-game lag as a result, although they have fixed some of the more glaring connectivity-related issues, such as getting stuck on invisible walls while transitioning between zones (when it shifts you from one server instance to another).