Halliday & Matthiessen (2004: 463):
… the imperative is a somewhat indeterminate category, having some features of a finite and some features of a non-finite clause. Similarly, the category of a reported imperative (‘indirect command’) is not very clearly defined. But non-finite clauses with to, following a verb such as tell or order, can be interpreted as reported proposals. They similarly display properties of ‘indirect speech’ …