If you have a lot of code that's never more than 80 columns, you would think you could set your terminal to 80 columns wide, open emacs, and never see lines wrap. Which isn't what happens. Instead emacs reserves the rightmost column to indicate whether a line is too wide for the screen. [1] This means you need an 81 column terminal to show 80 line text.

This is the only way to make the display unambiguous, but I'm not sure preventing ambiguity is worth it here. Why waste one column of every terminal to represent a rare case?

[1] It has two ways of doing this, one in which long lines get an $ in the rightmost column and extend off the screen and another in which they get \. Both need exclusive use of that column.