> The reason not to do this is observable in your post on my phone. The spaces cause the word wrapping algorithm to leave a dangling dash at the end of the line which looks ugly. Omitting spaces prevents the word break.

That's an interesting practicality but I don't think it's the cause of the rule: The rule probably long predates automated line breaking. Also, I think automatic line breaking will break compound words at the hyphen; it doesn't require spaces (which is also obvious from a software development point of view: the logic is relatively simple either way):

  Lorem ipsum dolor sit amet, consectetur adipiscing double-
  decker lorem ipsum dolor sit amet, consectetur ...