That is not really true. It IS implemented system wide, not on any app by app basis. You can copy text from every selectable text field in the system (the problem is that fields in the browser are currently not selectable, but that is the problem of the browser, not the copy/paste system). If you develop an app, it is already there, you don't need to implement anything.
In the e-mail, the text is not editable, therefore not selectable (not having non-editable selectable text boxes looks like current limitation of QtQuick, so they might have to implement it in Silica).

I don't see Android as a part of the system, but sure, it would be nice to implement some type of sharing functionality between those two clipboards (optional though - I don't want Android apps to be able to access my clipboard, I'm glad they are separated).