I think it's because the forum needs a 'plug-in' to work with Tapa Talk at all. The plug-in that this forum has is an old Tapa Talk one which still works but users are getting this warning about the message.
I have a very old version of tapa talk (I hadn't realised how old until this conversation) and it doesn't give the message with that. So my old tapa talk version works fine with the old plug-in on the site, but newer versions of tapa talk get the message when they find the forum has the old plug-in.
I've now tried TapaTalk 4 and got the same problem. That version is still in Beta and not yet released (they will be charging for it once it's out of Beta).
With Tapatalk 2 I don't get the problem. Not sure about Tapatalk HD (for tablets) which is due to be merged with Tapatalk 4 once that's released, as I don't have a tablet.
It looks like there are some options in the settings of Tapatalk 4 not available because of the out of date API, but otherwise it works.
Well, I bit the bullet for my iPad and paid the whopping fee of a few bucks, lol... I don't get it anymore for my iPad, but I still do on my iPhone. Perhaps it's just a Tapatalk issue?