Why does an exhaustively returning switch fail to compile if there is code following it?

Question

Why does this code fail to compile when using noImplicitReturns...

function foo(bar: "a"): number
{
    switch (bar)
    {
        case "a":
            return 1;
    }

    let unreachable;
}

error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

...while this code compiles with not so much as a peep?

function foo(bar: "a"): number
{
    switch (bar)
    {
        case "a":
            return 1;
    }
}