Just because your neighbor murders someone, doesn't mean you should, too.
In this case, the comment is really saying:
a) The parent comment suggests it's ok because other people do it.
b) The argument that things are ok because the other side does it is not a valid one however.
c) Take murder, for instance. If one side commited murder, no-one would consider it's ok for everyone to.
d) Hence, by analogy, the argument is incorrect.
I'm sorry for having to write this out, but this line of misreading does frustrate me. Surely it's possible to read subtext in people's writing, and not jump only to the literal meaning?