Earlier quoted context omitted.
I mean, in my case, I was literally forbidden from canceling. I got a screen saying I couldn’t cancel within 24 hours after the monthly renewal. Very clearly intended to combat “oh shit I need to cancel that one” when the charge shows up in hopes you forget again.
I'm trying to play Devil's Advocate, but I literally can't think of another reason they would do this.
While I never implemented a restriction like this, it would have prevented a lot of weird bugs/customer support issues and kept the underlying code much simpler.
(Annoying math = time zones, prorations, discounts, billing cycle anchors, etc. see the “falsehoods programmers believe about X” series)
(Systems to sync between = internal DBs, billing APIs, payment processors, etc.)