I think it's for a variety of reasons...
I use to do it, but took so much (you now what word goes here) for it, that I didn't want to deal with the FB messages, emails, and threats I received on the river any longer. I still do post the odd report on small U.S. based forums, where the people have no reason to be pissed off, and it's a much smaller community group that actually appreciates the reports for the pictures, story - rather than just trying to troll around the internet for a report on a hot river.
Another main reason I don't really post is the time that goes into it. I'd rather just post some nice pictures, and keep it at that. Even with detailed reports and descriptions, I'd highly doubt even 1 out of 100 people on here would recognize a bunch of the island rivers I fish (you'd have to have fished there multiple times to recognize any of them). That being said - you'd get that one guy who for whatever reason feels the need to post the river name. At that point reporting is a problem. Lots of the small rivers don't need the pressure and the poaching (which has been a HUGE problem this year on the island). I just avoid posting any reports, or even background pics of smaller flows online to avoid the potential name throw and subsequent consequences.
I do agree however... It sucks as I really enjoy reading reports on days when I can't fish or just have a break from uni, etc. As I said, I tend to go to smaller US based forums for that, as the reporting seems to be continuing there - and just the way I like it too, no river names, just a story and some pics!