If you just glance at the numbers it looks like they're being screwed. But at each round (while they still had control) the founders got to decide if taking funding would be better for the company and themselves.
There's no rule that says founders get to keep large percentages of their companies.
My point is that it doesn't have to be that way. The VCs are behaving in a predatory way, and they don't need to. I've worked at such a startup, and taking funding isn't just a nice option; usually it's a matter of survival. Of course a VC firm is going to take advantage of that fact. But if they showed a little more character by not sucking up such giant gobs of the companies, the founders would be encouraged to stick around and maximize the company's value. Instead, they're just encouraged to get what they can and then get out.
There's no rule that says founders get to keep large percentages of their companies.