Enough editors do it that they must think it softens a blow, but what is with the passive rejection?'
'We can't use your story'
'We are unable to publish X'.
Is there some force physically preventing the editor from writing a contract? No...and really, is that more polite than
'This doesn't fit our magazine'.
'I didn't like it enough'.
(I have a ton of respect for an editor who admits he rejected a story just because he didn't like it. Because that's honestly why a lot of stories are rejected. Editors just feel the need to rationalize it so people won't think they're biased...except that everyone IS biased. I sure am).