Halliday & Matthiessen (2014: 582-3):
Furthermore, causatives have passives; so we can have
(high) they were made/forced/required to accept
(median:) they were got/obliged to accept
(low:) they were allowed/permitted to accept
and this enables us to interpret modulation as it occurs within the verbal group:
(high) they are required to accept they must accept
(median:) they are obliged to accept they should accept
(low:) they are allowed to accept they may accept
Verbal modulation with must, etc., is now a kind of modality; it is semantically related to those passive causative modulations which have the circumstantial senses of ‘do under compulsion/from obligation/with permission’. What links this semantically to modality in the other sense, that of probability, is that both represent a judgement on the part of the speaker: just as in that may be John the may expresses the speaker’s judgement of likelihood (‘I consider it possible’), so in John may go the may expresses the speaker’s judgement of obligation (‘I give permission’).