dots in usernames verboten

I'm experimenting with LDAP functionality, bolting our standalone Chef server installation (12.2.0, plus some older versions I no longer recall) onto an existing Active Directory domain. The weird restrictions on usernames are getting in my way, specifically the ban on dots, like john.stange. That's a fairly standard corporate convention for usernames. What's the rationale?

Just so we're clear what I'm talking about, asking the REST API to create a user by POSTing the following to /users results in a 400 (Bad Request):

{
 "username": "john.stange",
 "first_name": "John",
 "middle_name": "LDAP",
 "last_name": "Stange",
 "display_name": "John Stange LDAP",
 "email": "john.stange@eglobaltech.com",
 "password": "sometestnonsense"
}

Strip out the dots (set username to johnstange) and it happily complies.

Start at the REST API POST /users entry point and trace the username validation involved in LDAP and Active Directory integrations. Determine where the dot restriction is enforced and clarify whether the intended outcome is permitting dotted usernames or documenting the rationale; done should include a decided behavior for the provided request.