It`s Free so what's to market; in a mixed environment passing malicious code via mail received on a MAC to a corporate PC, then subsequently to a client, is entirely feasible.
ClamXav has negligible impact on performance, not caring if your Mac passes on malicious code, is all fine and well if you work in isolation, in the real world it might not be considered such a great principle.
Q-6
Marketing something isn't always about making money cause that's what your implying.