I had a beaker-browser in mind, not safe-js. But that is also not true, I was just misled by names of files to download from safer-beaker-browser download page: there are .tar.gz and .zip files, which I assumed were source tarballs, and in fact they are distro - agnostic binary releases. So I was afraid that lack of package for my linux distro will force me to compile the browser.
Anyway, I found a thread about these security conerns: Public Notice - How to hack SAFE Browser Plugin users. It seems that main security problem is a possibility of SAFE users accessing normal “clearnet” (http://) pages, right? So to prevent that, you force them to use browser with clearnet blocked. It seems to me like “let’s forbid children to use knife, because they can hurt themselves” attitude. Eventually someone will want to open non-safenet link from safenet site and you cannot prevent that. They will just find another way to do it.