Somewhere since 1.9 or 1.10 craftbukkit started filtering out attempts to apply negative walkspeeds. In 1.8 and before it was still possible and even supported if one takes a look at the javadocs.
The responsible line of code:
player.abilities.walkSpeed = (Math.max(value, 1.0E-004F) / 2.0F);

This makes sure that the walkspeed is at least 0.0001, filtering out negative walkspeeds as well.

Is there any reason for this? Could this be solved and still allow the previously possible negative speed values?

Negative walkspeed values can be used for changing the players fov ('zooming effect').
Directly setting the walkspeed to negative values via nms seems to work just as before.