I have been criticised for answering off-topic "is this correct" questions, which I accept, but others are still doing so without censure. What is going on?
I followed up with a comment to https://ell.stackexchange.com/q/105179/ to clarify that not everything that asks "Is this correct?" is proofreading. There is a discussion about answering questions that need improvement in DO NOT FEED THE BEARS and I think that the comments were intended to "show you the ropes" so that we can be more consistent as a community and not to censure you. Let me assure you that you aren't being singled out.
There is an on-going debate about where we draw the line between making sure the folks get a useful answer to questions that are simple for fluent speakers to answer and protecting the integrity of the site as a whole to make sure that the content is high quality and useful. Our community is constantly evolving, so as frustrating as it is for the folks that have been here a while, the balance is constantly being renegotiated as we attract new members.
There are very few rules that are enforced, and they all deal with keeping the discourse civil. Everything else is a community-driven norm. Use your best judgement, be open to feedback from other community members, and be respectful when you disagree. It's difficult to not sound a bit harsh when you're writing a character limited comment, so cut folks a bit a slack and assume good intentions. If you find something that isn't civil or constructive, flag it and a moderator will handle it.
Our goal here is NOT to make sure everyone plays by the rules and to "punish" folks that don't meet quality standards. Our goal (as stated in the tour) is to build a library of detailed answers to every question about learning the English language. Sometimes questions need to be put on hold so they can be improved, and sometimes they need to be closed because they just aren't answerable in our format. We should keep in mind though that without questions to answer, we have no purpose.
As J.R. pointed out, there are a number of discussions on meta that illustrate that there is no hard and fast rule - here are a few related discussions:
Have we forgotten our mission?
Stop answering close-worthy questions!
Stop throwing "Proofreading" close reason at questions that are clearly limited to a single point of concern
Handling scant "What does this mean" questions
This community is not policed 24/7 by a small committee; it's made up of several people who have various interests and who are on at various times. Therefore, it is unreasonable to expect consistency in how feedback is handed down.
If you seem to be getting more direct feedback than others, then it might be that you're posting answers at a time when other folks who have this peeve are frequenting the site.
As a footnote, I'm not entirely convinced that the folks chiding you for answering off-topic questions are in the right. Some regulars seem more interested in policing the site than helping the learner. Judgement calls need to be made, and not everyone will regard a question the same way.