It is not illegal to use the bots, however it IS illegal to design any client that loads the RS.jar to RAM. By creating a client that loads RS, without the permission of Jagex themselves, you are legally breaking the Digital Millenium Copyright Act.
Some might say, 'Oh what about Swiftkit?' Well, when Swiftkit first came out, there were bans from players using it, and Jagex actually threatened Swiftkit with legal action, causing them to meet and Swiftkit to sign contracts and agreements with Jagex, allowing them to continue with the development of said client. Jagex obviously isn't going approve a Bot Client.