"The law says, 'Any person who leaves the country and later returns will be put to death.'"
They brooded on this in silence.
Finally, Manjiro said, "But why?"
"Because, if we were to encounter any of the foreign devils, we would be poisoned by them." (1.2.40-43)
Goemon and Manjiro are commiserating about how they might not be able to return to Japan. Why? Because Japan in the late 19th century is completely into isolationism, which basically means Japan wants nothing to do with any other country in this book. "Foreign devils" is some pretty strong language, though—it seems personal instead of political.