Even if you have, say, 5 sequential related structs, that are all virtually identical, all written on one line so that the similarities and differences are obvious at a mere glance... Then someone comes through and touches my file, and while they're at it, "fix" the line that went 2 characters past the 80 mark by reformatting the 4th struct to span several lines. Now when you see that list of structs, you wonder "why is this one different?" and you have to read carefully to determine, nope, it just contained one longer string. Or god forbid the reformat all the structs to match, turning a 1-page file into 3 pages, and making it so you have to read and understand each element of each struct just to see what's going on.
If I could have written the rule of thumb, I would have said "No logic or control shall happen after the end of the gutter." But if there's a paragraph-long string on one line- who cares?? We all have a single keystroke that can toggle soft-wrap, and the odds that you're going to need to know anything about that string other than "it's a long string" are virtually nil.
Sorry. I got triggered. :-)