You can be in a position where turning back will put you into Cb with the added hazard of reduced g margin so it is better to continue. For the non pro: carrying out a 180 at 36000 ft takes up a LOT of sky.
I've had Cb avoidance problems over China and just did what was best for my aircraft safety whilst 'discussing' the situation with ATC who was afraid that we were going to inadvertently enter Vietnamese airspace.