I joined a team working on a single page app that disabled text selection (via CSS user-select:none) to give it more of an "app" feel. While I understood disabling text selection on certain things, the "disable everything" approach drove me nuts.
When we had a redesign of our app, I decided that was one rule that could be left out (and only applied where needed).
I think it has more to do with apps which have some touch-tracking in place, perhaps for creating a heat map. Try copying text from a twitter post on the mobile web app or Facebook mobile web app..
I suspect it is this way because double tap was reserved for zoom-to-fit in iPhone OS 1.0, and now we're stuck with it being that way.