The bots they can detect with that system are cheap ones that run below the game code. Bots that run on Nexus are undetectable code wise by Jagex. But like matthuw (above^) stated, it is still very obvious to players who is botting. Also like he said if Jagex wanted to stop people they could, but they are making too much money from botters and RWTers.
Spot on. Money from bots spends just as well as money from real legit players. In the end, Jagex is a business, money is their only goal. Having the occasional mass ban and random bannings in between is merely for show. It helps keep the legit player population happy, while at the same time, they aren't losing any profit from bots and RWTers.
Like I posted in a previous thread. I bought the AutoSorceressGarden Pro bot from rsbots.net. Within the first ten minutes, I noticed about 3-4 things that could easily be used to flag anyone using the bot, without accidentally flagging any legit players. If Jagex really put in the effort and time, and just did a little homework, they could easily kill off botting.
If I was Jagex, I would go through the most popular bots, and research all the flaggable options of each script/bot. Then I would wait several months, simply only flagging the accounts, and at the end of the few months, do a massive ban. It's the most effective way to get all the mains, pures, skillers, etc., banned in one go who are cheating. If they just start banning people right away, people will know whats up and either stop botting, or update all the scripts (I haven't a clue why some haven't been updated already).
To be honest, I would love to work for Jagex, because I'd have one hell of a field day banning thousands of accounts.