Good catch, in your second scenario, when both speeds are 0 the code-path will return doing nothing. 
But this won't undo the current setting (1024 kbps), which is why you still see the throttling. In all fairness, I am not sure I can "fix" this, since your second command still had "throttling enabled". A fix would mean coding up "throttling enabled + both speeds 0 = disable throttling" instead of "throttling enabled + both speeds 0 = do nothing and return"
 

Once we make changes to CPanel UI to not accept empty values for download and upload speeds, I think we should be good.

Zhanlu, please update this bug when you make changes to CPanel UI. We can then probably close this bug.

This issue is already fixed. We now enforce admins to set a value if the throttling is enabled. For existing customers who have the policy enabled but no value set, an error will popup when the admin tries to save any policy change.