Unfortunately, I'm not very familar with SBE 2013 but I believe this is a new "feature" to check for the presence of another AV. I know if you have malwarebytes installed, the installer will also stop. This does not happen with SEP 11.x/12.1
I've got to believe it's checking for the presence in a reg key or possibly for a running service.
Aside from the opening a case with support to get more info on this, you may just need to meticulously go thru and weed it out, unfortunately.
Brian