Early check-in “hack” or just Marriott being nice?

I always book 2 nights when I take a red-eye to the east coast so I can check in around 6–7am and have a room ready. But almost every time, the front desk tells me they won’t charge for the first night since I didn’t “actually stay.” They just roll it to the next night instead. I’m Titanium, so I’m wondering, are they doing me a favor, or is the system just not built to check people in retroactively? Trying to figure out if I’ve just been lucky or if this trick actually works.