> Then why on earth is NULL being treated the same as false for the purposes of a WHERE clause.

Because, by definition, the WHERE clause returns a record when it evaluates to TRUE. FALSE and UNKNOWN are not TRUE.

Second, you absolutely can do what you're trying to here, but not the way you've done it:

  SELECT CASE WHEN bar = baz THEN 'foo' ELSE 'bar' END AS corge
  FROM quux

You're asking the wrong question and wondering why you're getting the wrong answer.