If I broadcast a request for an IPv4 address and your DHCP server proffers an IPv4 address that I can use for the next 15min, the address of a nameserver, and the address of a router that will forward packets to the global internet, I can reasonably conclude that you intended to allow me to exchange packets on your LAN and at least attempt to use your gateway to interact with the internet. On the other hand, depending on the situation, a "403 Forbidden" could reasonably be interpreted as a request to not send that type of HTTP request anymore.
The protocol isn't the only place to look for intent, but it absolutely does express intent in some situations.
If the owners don't want something public, it's trivial to lock it down--they might as well freak out when somebody uses the wrong door to enter the front of their shop.
Making a mistake doesn't relieve one of legal protection from trespass (physical or digital).
The protocol is relevant, because it conveys information. Just as unlocked doors generally indicate permission to access, unsecured HTTP generally indicates the same. But the protocol is only one piece of the puzzle. It is not dispositive. It does not conclusively decide rights and responsibilities. If a reasonable human would discern that the protocol allowing access was probably the result of a mistake rather than intent on the part of the property owner, that is what matters.