As far as I can see it's a bug.
SMF doesn't delete the topics reassigned to another used.
Reproduced in a freshly installed SMF 2.0 RC5.
The problem should be that the topics are not "reassigned" to the new user (id_member_started) and then when it's time delete them SMF fails because the id of the reassigned member is not the same as the id of the id_member_started now 0.
Edit
This query:
$smcFunc['db_query']('', '
UPDATE {db_prefix}topics as t, {db_prefix}messages as m
SET t.id_member_started = {int:memID}
WHERE m.id_member = {int:memID}
AND t.id_first_msg = m.id_msg',
array(
'memID' => $memID,
));
at the end of the function reattributePosts, just before the return should fix it.

(don't have any idea how it works on big databases!

)