Standing Reservations clarification please

The documentation, section Requirements for Creating Standing Reservations (UG-139, or page 1685 of the big book), states that

You must set the submission host’s PBS_TZID environment variable. The format for PBS_TZID 
is a timezone location. Example: America/Los_Angeles, America/Detroit, Europe/Berlin, 
Asia/Calcutta. See section 1.4.5, “Setting Time Zone for Submission Host”, on page 10.

Does this mean that PBS doesn’t honour the timezone setting in the OS itself?

It’s true - if you try to set a standing reservation without a TZ it throws the pbs_rsub error: a valid PBS_TZID timezone environment variable is required. error!

What’s the reasoning behind not honouring the OS timezone?