Thing is, there are clauses on both ends that prevent the verbatim copying of content over to Jagex's Wiki.
CC-BY-SA means you have to attribute the source with links (where possible), and Jagex's Wiki has a stipulation that all content you submit is your own.
If there is a ton of CC-BY-SA violations occurring on Jagex's Wiki, I expect some bad blood between Jagex and RSWiki at first. But ultimately they'll resolve it.
Yeah, I was going to mention this.
As soon as an editor on RS Wiki hits "Submit", their content has been 'given' to RS Wiki subject to a CC-BY-SA license. However, one of the conditions of said license is that although third parties are free to use whatever the license is for, the original author(s) must be accredited in any subsequent versions. Another condition (the Share-Alike part) means that if the content is used elsewhere, it must be released under the same CC-BY-SA license as the original work.
Well firstly, Jagex's Wiki does not allow authors and contributors to be named, therefore there is no means to credit someone else's work. Secondly, everything submitted to Jagex is protected under commercial copyright, therefore they're not publishing the information under SA terms.
Simply put, unless Jagex change the copyright status of information on their website, and until they provide a way of naming a work's original creators, they cannot
either themselves copy information from RS Wiki, or allow others to copy information to their website on their behalf, and if RS Wiki found a case of plagiarism, Jagex are legally obliged to remove it immediately.