This was intentional at the time of implementation, but it's debatable whether it's the proper behavior and there are good arguments for and against making it affect the mouse.
For: The flag name suggests that it would force strafing in all circumstances. If someone who plays keyboard-only makes a mod assuming this behavior, and then someone with a mouse just breaks it, that'd be bad.
Against: The flag was added primarily so that things like Axis2D could let you move by using the turn keys. Using the mouse to move in that would be strange, to say the least.
Do we have spare player flags available? (Time for PF2_?) The best solution might be to have PF_KEYBOARDSTRAFE and PF_MOUSESTRAFE which toggle the force-strafe behavior for each method of control, and then make PF_FORCESTRAFE an alias for PF_KEYBOARDSTRAFE|PF_MOUSESTRAFE. Then Axis2D can just switch to using PF_KEYBOARDSTRAFE and everything will be right in the world