And no other class has these such restrictions. A fighter doesn't loose any attacks if he decides to kick someone while holding anything.
Is this true? I mean theres all those rules about shields and which weapons you can use while holding a shield. Basically you cant fight hardly at all as a Fighter while you have a torch and a shield, right? "I put down my lantern and draw my sword", etc.
I'd certainly assign a penalty to a Fighter who tried to kick a foe while carrying a treasure chest in two hands. But, honestly, I'm not familiar with any special rules for kicking; he'd have to use Unarmed Strike and give up his regular attacks to make this kick? Is there a better way to do this?
As for the Monk, sure, he can use all of his body as a weapon, but fighting with one hand tied behind your back is still harder than not. If you hogtie the Monk, I certainly wouldnt want the Monk claiming he can still attack with his full bonuses, "Because my entire body is a weapon! I headbutt you!" ;-)
So its pretty clear there should be some penalty depending on how many limbs you have lost.