1. Rootpipe is a vulnerability in OS X. A malware may take advantage of this vulnerability and when such malware becomes known by Avast! they will probably add signatures for it, however, Avast cannot protect against, or patch the vulnerability itself, only block malware that takes advantage of it (of which there isn't any known implementation, AFAIK).
2. The person who found the vulnerability as only disclosed the details with Apple, as far as I know, not with Avast (or with anyone else willing to write malware or software to guard against it).
3. Because this is a vulnerability in the OS, it's up to Apple to patch, it's up to Avast to keep track of any malware that uses it.