I know, I've been through there several times, different hours of the day and night. It is always closed.
I have family in CBP and friends...believe me, they can only do what they are ordered to do. The bureaucracy of this organization is suffocating.
They were checking on I-5 last Friday night when I went through, and it was the first time in years that I had ever seen the PAL lane open. I've seen them checking late Friday nights, some Saturday mornings, or some Sunday afternoons. I don't usually go through on weekdays, so I wouldn't know.
But, I've never seen them check (so they're closed?) during the higher traffic "rush" hours, when checking would cause miles-long jams, nor have I seen them check when traffic is too light to form a queue (which could be really dangerous for them and for drivers).