Because it's not a storefront to sell apps, it's just a catalog, and the developer of the webapp needs to take care of anything like that on their own. The submission/review for these is basically a cursory look to see if it's actually an iPhone webapp, categorized correctly, etc., not much more. How would Apple try to cash in on these when most of them are free or tied to a service that I assume a lot of developers created the webapp for in the first place, rather than have to fork over 30%/pay the $99 iOS Developer Program fee/deal with review hassles with Apple?