It's because it's logitech. No logitech I've used so far is easy to use.
Simple fix though.
Go to Start -> All Programs -> Logitech -> Settings. From there, just find the button you want to remap and select "keystroke assignment", then enter the button you want under "specify keystroke".
For example. On BF2, I have tilt left as knife, and tilt right as pistol. So, I remap tilt left to '1', and tilt right to '2'. This will disable their previous function as being able to sidescroll, but it's for a good cause.
By the way.. I have a Logitech G5 aswell, all I can say is it's an AWESOME mouse!