Following this argument maybe Apple should also start review websites, and only allow the known good ones so you and your iDevice are not damaged by evil / malicious sites! As HTML/CSS/Javascript could 0day hack into your whole phone this is a security disaster waiting to happen! Get some more censors, fast !!
They could do, but websites are out of Apple's jurisdiction so they best they can do is tighten the security of the browser. What they can also do is make sure 3rd party browsers don't contain any exploits that could obtain system access.
They could do this by, oh I don't know, vetting and reviewing all 3rd party apps using a walled garden approach, for example.