Really, nothing mysterious about it.
Out here on the 'Open Forums' or otherwise known as the free section.... you have all kinds of lurkers, nose pokes and other tire kickers that don't want to know about the real make up of the sign industry. They're just looking for ways to get around local sign shops, cheaper shops and DIY-ers that hurt the business end of things. In the paid section... people will open up more and talk about much much more than out here.
Many people on any given day are turned away as the administrators monitoring [can attest to] .... because they are not interested at all in furthering or helping their fellow sign people. They tend to be the 'Takers'. All they do is take-take-take and don't' give back. Don't get me wrong... someone new that is really trying will take and take and take, but they are educating themselves and when they do it in the 'Premium Section'... they are at least acknowledging they are willing to learn with the rest of us.
Again, there's nothing wrong with the free section, but some are hesitant to reveal trade practices, pricing and other valuable things which can undermine anyone's business out there anymore.
Let's take for instance someone asks a vital question and it gets answered by someone here. The answer might very well help that OP, but at the same time, it could hurt some other sign shop four states over from yours because some lazy a$$ed customer stumbles on here while googling vinyl made banners. They see a way to get information so they can go to their local sign shop and demand why he is over-charging or whatever other accusation they dream up... and that signshop guy/gal might be green around the gills and lose a customer by being blindsided.
Okay, now that's rare, but some of my customers have asked me if I'm on this site and I tell them, Yes. They've actually asked me why so much information is passed around for anyone to see ?? For that, I don't have a reason, other than some people just plain don't care.