Simple, the PM_Users table only has a number to identify which pm's are which pm's; there is no content to identify which is a "welcome". You could take the id from the other tables, but if you're doing a broad delete with an sql statement, i highly doubt in your ability to log every private topic id to be able to purge them from the database.
The php script opens the private topic table, grabs the id, and purges that id in all 3 tables at once, then continues; all done on its own, and messages purged from all 3 tables.
Simply put, you can't search "welcome" in the _users table as all it contains is numbers.
