You're allowed to do much worse to a moser. He's the textbook example of one.
"textbook example" of a moser?
I'm not trying to defend him, but do you know of a specific case in which he called authorities and reported on someone? Yes, he posted damaging information in public, on his blog, and that MAY constitute mesirah, but I'm not certain that the label of "textbook moser" is an accurate depiction.
As an aside, the "textbook moser" may not even apply today, as it's clear from certain halachic works that the fear was that mesirah would lead to the government killing Jews, which is a long strech today. (I can cite sources if you are interested in the technicalities.)