M+ and M- are simply the outputs of the victor speed controllers. Motors can be given polarity in either direction, it just causes a reverse in the motor. This is a proper way to do it and GOOD PRACTICE. It allows your programmers to give equal control to the motors when they are running the opposite directions.
If you are interested, read up on motors:
http://www.howstuffworks.com/motor1.htm
This will explain that it is OK to reverse the polarity on a motor, and in fact the ONLY way to drive it in the other direction.
Another way to do it is through the programming, but switching your leads is the easiest and gives you the most even control.