All my friends have emails, and most of them have Facebook, but not all of them check Facebook that often or only have Facebook to not become a social outcast. You either need to contact them by phone or by email if you need something from them in some immediate time frame. Add to that the fact that there are more emails listed on the site than phone numbers so your primary option is email them.
So now those people who haven't bothered checking Facebook cannot be contacted because that email address just routes back to Facebook. It negates the whole point of having your email listed, which is for your friends to contact you in some non-Facebook way.
And Facebook just has a fetish with mucking with your personal settings. It's the only company that routinely does this kind of thing without bothering to say anything about it.