Question about replying to job offers

I was just wondering something about the content creation section. Is it okay if we reply to the post offering the job and saying that we can do it and requesting further information? I have seen thousands of posts of the exact same sense joining up so I figured we could but I just received an infraction for doing it which I do it rarely (usually PM) but I'm not complaining about that, I just want to know for future referrence.

In what situations is posting allowed because in every situation the person could have made the enquiry via PM including the threads where fellow writers are offering their service and not just those looking for a writer.

Thanks.