Halliday & Matthiessen (1999: 526):
Although these are derived from the sense of proposals (‘you are required / supposed / allowed; I am able / willing’), they are not restricted to clauses having these speech functions; obligation and readiness are construed by the grammar propositionally and hence are used freely with third persons. But they still represent the judgements of speaker or listener on the obligations or inclinations involved (he ought to help, she will help).