You're not mistaken; it does. However, for each database, a user is required to create another username to be associated with that database. It'd be nice if we can choose not to create an additional username for each database.
The only reason otherwise that I can think of is a little bit of added security (I do mean "little") It could possibly make it more difficult for hackers to gain access to multiple databases if they were able to get it, or at least compartmentalize damage to that single database and not all of the user's database. But as long as it's limited to their databases and was caused by a script flaw and not a security issue with the server, that's their choice...