Yes, this is confusing. Being a "long-time" Dreamhoster, I was frustrated by not being able to have a common user name for different databases,.
Suddenly, one day, the system started letting me assign a user to more than one database, and I was happy.
Now, back to the "old ways", which is a bit of a pain trying to keep track of which user is assigned to which database. The method I have used is to just add a prefix or a suffix (in a systematic way, of course, to make it easy for me to remember but not discernable by others - I hope) to the existing user name I would have used if I was allowed to.
A bit kludgy, but workable for me. I really did like being able to have the same user for multiple databases, though!