Sprite Rotation does not have an effect on the width value because it shouldn't. But open Photoshop and slowly reduce the width of an object, once you reach about 1% width, your object is a thin line and at 0% it's gone, once you reach -1% it's at the opposite/mirrored direction.
This is the same for every software I know, even 3D Modeling software flip the scale of XYZ when you mirror them. -1 Scale in any of these axis gives you a mirrored object.
If you want to use your width in a positive value, simply use abs() as you have stated. It's a little bit tedious, maybe, but trust me, the uses of knowing that your object is mirror through the negative value of the width is a much better and will also fundamentally help you in many ways.