While SETI should use the default "not alien" in its results, it seems irrational to denigrate the possibility of extraterrestrial microbial life on Mars (or anywhere else).
It should be remembered that the Viking LR experiment yielded results that showed the existence of biological activities according to the experimental design. Since then, other non-biological explanations have been proposed.
Also, the Martian meteorite AHL84001 had morphological features which were described as biomorphs. Since then, other non-biological explanations have also been proposed.
I won't speculate on why the Martian research community speaks as it does but it would seem that allowing for a biological, as well as a geological explanation would be a better scientific approach.