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.
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.