I don't personally have issues with jealousy in polyamory at this time, but I don't think it's bad that people do. They post here because they want to work through it. They don't take the position that if they're jealous, then they're right, and the situation is wrong, and poly is wrong. They try to figure out what causes the jealousy and they fix that.
I have 3 brothers and as a kid, there was jealousy all around from all of us. But we knew it wasn't right to be jealous of your siblings so we worked it out. Just knowing it didn't make it go away though.
Some people are more jealous than other, but being fit for poly doesn't mean you never experience jealousy, it means you know how to deal with it and move past it. I think acknowledging one's weaknesses is a form of maturity, and the people who post here and admit that they are jealous are showing aspects of themselves that a lot of people would want to hide. But they put it here for all of us to see so that we can help them through it. I don't think it's immature.
Also keep in mind that some threads have a venting aspect to them. People who have no place to talk about these things (mostly because people would probably just tell them "he/she is cheating, dump them!" or would say "well it's your fault for being poly" or just wouldn't understand at all) come to the forums, and here they can talk, and sometimes it all comes out in ways that aren't necessarily pretty. But then you can sort through the feelings and see what to bring up to your partners, instead of having that explosion of feelings in front of them and compromising one or more relationships.
Some people can analyse their feelings and have calm conversations with their partners and sort everything that way. These people have no need to post about it here though. So of course the people who do post are those who need some help dealing with their feelings and want an outside perspective to know where the line is between jealousy that should be worked on, and feelings that just mean someone is treating you like crap and you should confront them.