Considering it’s a API available without any authorization, the better comparison would be walking around on unfenced private land. There’s nothing to indicate they don’t want people on it but it’s also obvious it’s private land.
It doesn't matter. It's still just as illegal to get into an unlocked car or one with wide open doors without permission. The same premise applies to computers in a lot of places, access controls don't matter. If you access something on a computer not indented to be accessible, it's considered a crime.
Is it illegal, in fact? If a cop saw you, you'd be arrested and prosecuted for attempted auto theft, and your "I just wanted to see how comfy the driver's seat was" defense would ring hollow in court. But sitting in an unoccupied car without authorization isn't trespassing unless it's parked on the owner's land, and I'm not sure what other laws would apply to that specific act.
Walking around isn't usually a big deal until told to leave (verbally or by way of conspicuously posted signs), since that is a prerequisite to trespassing. Otherwise, delivery people would operate in a gray area which would be very problematic for them, since not all deliveries are requested by the recipient/owner.
However, although you are free to walk around in search of the front door, you can't start eating the fruit off the trees. Perhaps that's the better analogy: the trees are happy to serve up a delicious treat for anyone requesting something of it, but that doesn't mean the tree sets the rules. Just because fences preventing this are popular doesn't make them compulsory.
