Posting should not be the only metric for activity.
Maybe not, but when you're dealing with an online forum such as this, it is one of the easier ways to not only gauge someone's activity, but also to differentiate accounts made by actual people versus those made by spambots versus those that were made in the past but that are no longer active. Now naturally, that does not mean that all accounts here with 0 posts are spambots, but just logging on and looking at stuff isn't really being an "active member". It's easy for someone to log in and leave their account logged in all day even though they may not actually be at their computer/not actually browsing the forum. Being logged on is not the same as actually being present and active on a forum. If you're logged onto the forum for 24 hours, the system will detect and show that your account was logged on for 24 hours. However, if you were only actively looking at threads and posting for 1 hour out of those 24, then you were really only on the forum for that 1 hour. They are not the same thing.
To use a different analogy, if you go into a shopping mall and spend all of your time there going in and out of shops just looking at stuff, but not actually purchasing anything, you're not "shopping". You can be there for 3, 4, 5 hours, whatever. The fact of the matter is, if you don't walk out of that mall with a bag of stuff that you purchased, then you didn't "go shopping" at the mall, you just "hung out" at the mall. It doesn't make you a bad person or anything, but you can't say that you did something when you were really were not.
Also, it's not like your posts have to be essays like you would have to do in school. Like I previously said, there are numerous threads here on a wide variety of topics that anyone can participate in. Are you a sushi or ramen fan? We have threads where you can say "Hey, I like
*insert favourite sushi/ramen type here*". You like movies? There's a whole section here devoted to numerous topics/thoughts on movies. Are you a sports fan? We've got a section for that too, where you can discuss your favourite teams/leagues/etc. You like idols/music in general? We've got threads for dozens, if not hundreds of idols/music groups. If we have a thread for an idol/group that you like, even just something like telling us your favourite song from them would be okay. Like
Stryfe said, as long as it's obvious that you're not just spamming the board to get your access back, then you'll probably be fine.
In the end, as much as you may think that it sucks/is not fair and as much as you may want/choose to protest, this is the new policy that the staff here has chosen to have, and as such if you want access to those sections, you're going to have to proverbially "bite the bullet" and take part in some of the forum discussions so that you can get it back. If you still don't like it, well sorry, but that's how it is now. You're free to go elsewhere to get your gravure fix, no one is forcing you to stay or keep coming back here if the forum is not what you want it to be.