If we consider malicious actions (like a big war), such colonies on Earth are just too convenient for military installations to be left untouched.
Mars colony is too far away to be able to deal a sneak attack; so wiping it is not as urgent, and after 3 hours there are no rockets anywhere on Earth for next 5 years.
Also, I am not sure that the cheapest way to "filter CO2 and CO, boost O2" is more expensive than "filter all possible chemical weapons and kill all designer bacteria".
Whether a war that realy destroys every human (maybe not immediately, but over the range of a century) is likely is another question.
Of course, intact Mars colony would also have to keep quite a lot of physics, engineering and biology knowledge even if some of it were lost on Earth.
If we can transport 80,000 humans to Mars and keep them alive there long-term, we have the tech necessary to wipe out those Martian colonies as surely as comparable ones on Earth.
Article on space.com