Halliday & Matthiessen (2004: 632-3):
In addition to metaphors based on ideational projection, there are other kinds of metaphors of mood as well. One prominent type involves a shift in the realisational domain of commands from ‘imperative’ to ‘indicative’ clauses. The ‘indicative’ clause can be either ‘declarative’ or ‘interrogative’; … The ‘indicative’ realisation of proposals has the effect of blurring the line between proposals directed to the addressee and propositions about how the world ought to be.