Detecting bots isn't that complicated... If you guys knew something about programming you could probably understand how easy it is to detect the kind of bots that are used for RuneScape then to flag the accounts using those bots. It's easy as pie (They also never hired someone specialised in catching automated software because it isn't that difficult to figure out a solution

).
F2p Accounts always get banned sooner or later, you can not avoid ban on F2p.
Jagex knows who is botting and who is not, no matter how complex the bot is. They see you that you are botting and they decide that you are not planning on becomming a member any time soon and so they get you banned. That's how they work.
If you are p2p, Jagex also knows that you have been botting, they are just letting it slip and do random roll backs for public eye. If they roll backed everyone, even tough it would be a good thing for RuneScape, everyone (p2p gold farms in china) would stop botting and playing all together, losing a lot of money.
So:
Their practice is to SHOW to the public that they do something about bots, but they are not powerful enough to handle all of them. But in FACT they are fully capable of stopping all botters once and for all, be it F2p or P2p. They just don't want to, since if they did, they would probably lose a lot of $.
Banning F2p bots, is really a cool strategy if you think about it. Botters will never stop, once everyone who bots figures out that being caught botting in p2p is quite rare, they will purchase P2p for all of their bots. If one person bots on 20 accounts, that's 100$ only from one guy.
So don't cry if you get banned on F2p, it's all part of their own strategy. If you want to bot either go P2p or don't bot at all.