The problem is that Google nominally operates at two levels of abstraction — one is showing you the map, of that's all they did there would have been no issue — that's parity with a paper map & up to the user to navigate.

But at a higher level of abstraction, they provide directions: basically "outsourcing" interpretation of the map to them. Here there's definitely an onus on them to say "Sorry, no route found" and _tell_ the user they need to interpret the app themselves.

By suggesting a route, they're obviously suggesting it's a safe/viable route. If they can't guarantee that, they should err on the side of caution.