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.
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.