This is more of a cultural thing. Editing articles themselves is carte blanche of course, but editing people's comments has a stigma. From a pragmatic standpoin

This is more of a cultural thing. Editing articles themselves is carte blanche of course, but editing people's comments has a stigma. From a pragmatic standpoint it probably prevents petty squabbles over typos, as well as passive-agressive tweaking from editors. On a deeper level though, it's about integrity of thought. Even for articles it's expected that quotes should present what was originally said. People can take it really personally if you change what they said and it's not hard to change the tone or force of an argument by changing even a word. Btw, I highly doubt it was an admin that gave you that warning. It was probably a regular user.