Often "units" may be used instead of degrees for simplicity and robustness of a display.
To take another example, on aircraft where the trimmable stab range varies over the positive and negative ranges, someone intending to set +5 and actually getting -5 will be seeing "-5" on the display instead of " 5". That could be missed.
If instead stab position is displayed as a always-positive scale, there is reduced scope for confusion and not seeing that small, but important, minus sign.
For the F-15 example above, the fact that the units display is always high by a magnitude which is probably close to a limiting negative alpha of -10 degrees suggests some of the same "never display a negative" thinking played a part in the design.