Having worked in the hotel industry for 10-years and also an avid RV'er for 18-years, I believe there's more similarities (conceptually) between campgrounds and hotels than there are differences.
The bottom line is both offer a resource in which the customer will occupy a space for a given period of time. The check-out time is critical to allow for preparing the resource for the next visitor. Hence, the check-in time standardizes the expectation, for the incoming guest, of when the resource will be available for their use. But, just like you probably experience with traffic laws, we never enforced either policy with an iron fist.
Honestly, when a guest arrived early, if a room was available, I was more than happy to check them in and hand over a key. We both won, because it was one less check-in I would have to handle later (during the busy time) and the guest could get unpacked and enjoy their day.
Late check-outs were rare, because most people were ready get to their next destination, whether it be home or somewhere else. However, we would accommodate such requests when possible. We never really asked, or even cared, what the reason was, because the result was still the same - use of the resource for an extended time.
In our own travels, we keep things simple and don't sweat the small stuff. Sometimes it's easier said than done, but we try to follow the practice.