This is discipline, in addition to the command to think well of people, i.e., if something unbefitting is mentioned about good people, then one should think well of them, and not feel towards them anything but good. Then if a person has any unsuitable thoughts about them, insinuated into his mind and imagination by Shaytan, he should not speak about that, for the Prophet said:

Allah will excuse my Ummah for anything that occurs to their minds, so long as they do not speak about it or act upon it.

This was reported in the Two Sahihs

[note: I thought it would be nice post the hadeeth in Arabic also, but unfortunately blogger doesn't seem to take them, it just printed out a question mark for each letter]