My understanding was that Safari used JIT which is prone to security issues with how it executes that code.
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.
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.