Most autoresponders send highly predictable responses that can be reliably filtered on.
And when that filtering fails, causing an email to be filtered out as an auto-response even when it was written by a "real person"? What happens then?
1) This is rare but unfortunate.
2) Autoresponders are almost totally useless these days.
3) If this was handled like spam (at last by Google), it probably would not be a problem.
Well, I guess it depends what level of false positives or false negatives you can live with. I think you might be surprised at the diversity in email autoresponder formats, though.