it would be interesting if they elaborate on those...
As to why it's safer to do so in Safar that in a webview must be because the only API to safar is from the JS/HTML/CSS but with a webview you have a C/ObjectiveC API from the other side, which may be harder to protect against.
[EDIT] - More information on the topic from Gruber http://daringfireball.net/2011/03/nitro_ios_43 although there is no reason given as to why a UIWebView is more at risk from exploits than Safari.
This reeks of the same logic that caused them to disable HTML file upload buttons (there's "no" filesystem -- which is apparently why you can't even browse for photos and uploads until recent IOS).
This is what drove "there's an app for that" for so many years: Apple's systematic limiting of web apps into a severely curtailed walled garden so that the app store would -- in fact, could -- be the only source for real applications.
"Web Apps aren't as performant as native" is a direct result of such strategies.
In other words, those strategies WORKED. As with all anti-competitive practices, they will keep doing their job for a while, until something new and better (or at least workable) comes along.
