the function "Call Forward" allows anyone to route any number to any phones. This requires that every Extension is absolutely trustworthy which is impossible in nearly all cases.
A employee redirects the Extension of his Boss to his phone and tricks the Caller into giving him confidential information.
A wife suspects her husband of cheating. So she redirects his calls to her phone.
It would be extremely useful, if there were more options to tweak this behaviour. But in the current state, all that is left is to disable it to prevent abuse.
In my opinion, the best solution would be a list of extensions, that are allowed to use this function with the own number. So the Boss Secretary could redirect the calls of the boss to his/her phone if he/she has a meeting. Another solution could be to restrict this functionality to the own number, but this would destroy many possible applications.