IIUC, it's a religious/cultural observance that's very important to them, I think a kind of reminder. I'd say respecting that means respecting that.

Also, I loved B&H when I was seriously into photography, and even if the closed days had been a practical inconvenience (they weren't, IME), it would've still been worthwhile.

Speculating... Maybe something in the culture of dutiful adherence also helped them to provide such well-respected service at great prices? Diversity is good.