Many modern mice have a horizontal scroll built into the scroll wheel. To activate it, you just "tilt" the wheel left or right. This is the case with most designer/gamer/wireless/high-end mice.

I own such a mouse (Logitech M305) and it is quite annoying not to be able to disable horizontal mouse scrolling because:
- Horizontal scrolling is rarely needed
- Most applications have shortcuts for emulating it (ex: shift+scroll)
- Middle-clicking...
   - triggers a horizontal scroll unless you are *very* careful
   - is used all the time for opening and closing tabs in various apps
     (Chromium, Firefox, GNOME terminal, gedit, etc.). In this case,
     it usually means that the app switches tab (or does some other 
     unwanted behavior).
   - is used a lot while designing (in Inkscape, GIMP, Blender,...)

All in all, it gives a feeling of increased strain and reduced precision.

This should be handled by the GNOME control center because:
- You're not supposed to use xorg.conf anymore, so there is no way for users to work around this problem easily (I don't even know how myself, Google failed me)
- It should be as easy as disabling the touchpad's horizontal scrolling