One supposed explanation is that Apple only provides beta stuff (and docs for betas) to paying developers to weed out people who aren't serious developers.
I would argue that it's not Apple's role to weed out subpar developers. It's the market's role to do that.
Strategically, Apple doesn't want anything on their platform that doesn't fit in. Nintendo and Sony are exactly the same with their consoles.
Nintendo and Sony have formal, up-front approvals processes for their respective consoles, which - if controlling what goes on/does not go on the platform is the goal - seems a better way of going about it.
Incidentally, SCEE has quietly dropped the approvals process for PS2 games in Europe.
They could also be trying to avoid tons of poorly built apps that overwhelm the market. I seem to feel that this is somewhat the case with iPhone apps.
Or are poor.
True, but only at the beginning. I think if you were a great programmer, you would likely build something that made money and you would no longer be poor. This does seem like a chicken-or-the-egg dilemma, though, by not being able to develop something without easy access to docs. Apple Developer Connection is free, however.