All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The Prophetic narration you referred to in the question is narrated by ‘Aa’ishah . She said: "Never that the Prophet, sallallaahu ‘alayhi wa sallam, had to choose between two matters, but that he chose the easier of them as long as it was not a sin (i.e. a prohibited action). If it was a sin, he was the farthest of people from it. The Prophet, sallallaahu ‘alayhi wa sallam, never revenged for himself unless the limits of Allaah were violated. Then he took revenge for it for the Sake of Allaah." This narration is reported by Al-Bukhari in three different books: in the Book of Virtues, chapter of the description of the Prophet, sallallaahu ‘alayhi wa sallam, and in the Book of Etiquettes, chapter of the Prophetic expression “Make things easier and do not make them difficult”, and in the Book of Hudood (plural of Hadd, i.e. a corporal punishment legislated by Islamic Law), chapter of implementing the Hudood and taking revenge when the limits of Allaah are violated. This narration is also reported by Muslim, Abu Daawood, Ahmad and Maalik .
The meaning of the narration is that when the Prophet, sallallaahu ‘alayhi wa sallam, was given a choice between two things, and one of them was easy and the other was difficult, he would choose the easier unless the easier of the two was a sin, i.e. a prohibited matter. If so, he would refrain from doing it. The scholars said: 'It is probable that the choice was from Allaah, and Allaah gave him the choice on things which have two punishments, and so, he, sallallaahu ‘alayhi wa sallam, would choose the easiest punishment. Or that the matter could have been in relation to him, sallallaahu ‘alayhi wa sallam, and the non-Muslims, about fighting them and taking security taxes from them, or in regard to his nation, in striving in acts of worship or in being moderate and he, sallallaahu ‘alayhi wa sallam, used to choose the easiest in all these matters. '
As regards the expression “as long as it was not a sin”, it appears that this is when the non-Muslims or the hypocrites gave him a choice. However, if the choice was from Allaah, then Allaah does not give a choice in what is sinful.