Certainly cancelling IS better than a no-show, provides a bit of a heads up. And a 45 minute advisement is better than nothing. Still, as wilisno points out, that slot might not be filled by a walk-in or a last minute call. Very real potential of lost income for both the girl and the shop. I doubt a shop keeps a list of standbys so that they can call and advise a punter of a cancellation. If you can comprehend the ramifications of a no-show, seems reasonable to think you'd also understand the ramifications of a last (well 45) minute cancellation.
Myself, I've had to cancel 3 times. 1 due to a positive Covid test, sent a message to the girl the night before, though, based on her response, she hadn't read until the morning a couple of hours before the booking. Likewise, another notification sent the night before for a late morning booking, and, again, based on her response, she hadn't read until an hour before. Worst case, I was literally on my way, 30 minutes out, when I was notified of a critical situation that I had to handle. Messaged the shop, quickly acknowledged. In all cases, they were gracious (2 regulars, the last a first time shop+girl). But like I said, I wouldn't take offense if they hadn't been. It is BUSINESS, and like an airline seat, if not sold, it's lost income once the time has passed. Seems a businessman would understand that and understand the pressure of keeping the cash flowing, both for himself and his employees.