Labella had some problems with flats breaking when strung through-body, but the last I heard of it actually happening was years ago. They are probably just playing it safe now.
Yea, there's some warning sign on the La Bella Jamerson Strings Box that I bought some years ago that told not to use the strings with a string through body bass, which may break the strings or something.
But as I said, I'm glad, that I never experience bad things like string break etc while using them through body.