An unfounded assumption. I type out URLs quite often on things like blog comments or when creating accounts on new webapps. I'm not sure why I'd go to my own Web site or Twitter page and copy/paste the URL when I can type it a lot quicker.
Further, you don't have to "delete" the existing "http:// when pasting. Just double click in the field to have all the text selected, then paste. It replaces the existing contents. Adds the time it takes to do a double click or press Ctrl/Cmd+A.
The problem with UI suggestions is that even when they're logical and seem to make sense, they won't for everybody - especially not those who've become so used to working around the problem, that a "solution" could expend more mental effort than following how everyone else does it already.