Those ICBM silos are in the middle of the country. When you launch an ICBM, rocket stages are going to be dropping on land, perhaps in inhabited areas. In a nuclear war scenario, that's about the last of your concerns. However, in a conventional war scenario, you have to worry about possibly killing your own people (or Canadians, depending on the trajectory).

One possible but unlikely solution to both problems (identification of nuclear verses conventional and dropping stages on friendlies) would be to build a few new silos near the coasts. Arm those silos with conventional warheads and allow for inspectors to verify this. Any launch from these silos would be known to have conventional warheads and they wouldn't be endangering friendlies.