Halliday & Matthiessen (2014: 178):
Thus once a proposal becomes discretionary, it shifts into the indicative mood to accommodate the modal operator; this also means it take[s] the full indicative person system, not the restricted person system of the imperative. Modalised clauses are thus in principle ambiguous as between proposition and proposal: this is shown up when the experiential meaning of the clause points strongly in one direction or the other, for example, she must be very careless is likely to be interpreted as proposition (modalisation), because one does not usually enjoin people to be careless, whereas she must be very careful is more likely to be interpreted as a proposal (modulation).