I think the -50/-75 to movement was added because originally (RM2), movement was the only action (other than parrying at half OB) allowed when stunned. All other actions were forbidden.
It might also be helpful to note that the original description of the Stunned Maneuvering skill in RM Companion 2 specified a -10 penalty to the skill roll per cumulative round of stun. Companion 2 also (confusingly) had its own Static Action table which noted both the -10/round penalty and gave additional difficulty penalties for number of rounds of stun accrued. E.g. 2 rounds of stun made the base Static Action manuever a Hard (-10) check. I say this his confusing because these difficulty levels don't line up with the simple -10/round that is mentioned both in the verbal description of the skill and in this very chart I am referencing on p. 92 of Companion 2.
Let me check that, I happen to have a copy of RM Companion II.
Well, Hurin is right. At least for the original description of the skill.