That is a intented implementation in PPM. This is mainly used by PM's to track the progress of plan as well before approving the time. If Manager is not fine with the progress of the plan he/she can reject the timesheet and thus rolling back the plan progress.
As soon as resource has a time sheet with time, it gets shown up in PPM plan and PM's can see that this many hours have been entered for the task. Now he can as well see the progress of whole plan and based on PM's wish can approve or reject to make time pertain or rolled back.
To add to earlier post , this setting cannot be changed. As a best practice, it is suggested to the customer to put a process in place and ask users to put in time on a particular ay of the time period configured. Thanks, Aravinth