ISBN is different: the parser deliberately tries to
detect text that the
user typed naturally ("ISBN 123456789" being the normal, unmarked formatting
used in the real world) and treat it specially. It's not a
real grammatical feature, it's a deliberate effort to achieve markup with no
effort.
The only other feature I can think of that works that way is bare urls:
http://foo.com
Anyway, it's not a major issue. There are bigger fish to fry.
Steve