Wiki markup is recognisable with link conventions [[like this internal link]] and [[like this|http://external/link]] (MediaWiki and Creole; earlier wikis relied solely on CamlCase). Markdown's design is inspired by conventions used in plain-text email, with links [like this][1].
Anyway it should still understand links without http because if I am typing the link out by hand (say because it is a site I remember) then I would still want it to be a link.
This is baffling for me. You are not new to the Internet, but until only quite recently, every single browser always showed the protocol is the address bar. Only of late did Google Chrome and then e.g. Mobile Safari remove “http://” from the display to save space; and yes, naturally they include this string when copy-pasting as doing otherwise would result in an incompletely pasted URI (protocols are not optional, except by current convention in non-hypertextual media).
[1] http://site/