You have to remember that this is the internet (and r/o b4 you comment ;-) and some people use it to say and act a way that they feel that they can not in real life. This is their chance to be b*tchy because the don't have the cajones to be that way in person. People tend to say things that would have gotten them beat up in real life.
Most people i know are the same irl and online but there are quite few that use this as their opportunity to be evil, mean, rude and downright stank.
Also don't forget that when you post something on any lj community that is not marked as private people can see it even if they are not in that community. There is a section on the front page of lj that lists the most recent posts. So if you post a FOTD (even in your own journal) that is public then some random person can see it and comment. Some people just sit and watch the 'most recent posts' section and comment on stuff they see - so the person may not even care about makeup or care about hurting your feelings. I don't know if MAC lj allows this but so many other communities don't even know that the section exists.
Most of the people on this site know how hard a flawless makeup is and would never say something so stank to a fellow specktra-ite. We are all here cause we love make-up. That is our bond and we choose to respect each other's skills, wishes to learn and attempts.