They're trying not to do so much damage to day-ticket purchases, especially as popular as those days are at DLR. Fridays/Saturdays are heavily those. But a day-ticket purchaser might balk on buying a Friday or Saturday where the park closes at 6pm. Since the other days have a higher percentage of AP guests (and a lower percentage of day-ticket buyers), they can move AP availability around to make room more easily. (I think day-tickets are bigger on Sundays at DL compared to DCA.)
The other So Cal parks are generally happy to get anybody through the gates on any given weekend day.