First of all, let's establish the proper context. I believe that the "user ID" to which Mr. Semenzato refers is a user ID which (in conjunction with a password, of course) is intended for use by a customer in accessing the former "MySBC" website which is now called "AT&T Account Manager". It's not an email ID or anything like that; rather it's just an internal identifier local to AT&T's web server. Some of these sites require that the customer choose a user ID while others have been designed to utilize the telephone # NPA-NXX-XXXX as the "user ID". For example, legacy AT&T customers (e.g., not pre-merger SBC like Mr. Semenzato) such as myself log in with our phone number and password to view our latest bill and so forth. It'd be just as simple (maybe simpler) for SBC to have assigned some arbitrary user ID to each account when the customer originally registered for on-line access but they probably figured that a customer would be less likely to forget an ID which he chose himself ... also this would make the login somewhat more persistent and 'portable' if the customer moved and got a new telephone number (yes, I know about LNP).
AT&T (nee' SBC) is certainly within their rights to constrain the selection of particular user IDs in any manner they wish ... similar to what is done with the choice of passwords. In the case of passwords, such restrictions are usually applied with respect to the number of characters, mixture of upper and lower case, no easily identifiable text strings, and so forth in order to ensure "strength".
In the case of user IDs, any restrictions (besides those which are purely programmatic like maximum number of characters) probably center either around uniqueness (for obvious reasons) or good-faith [although perhaps a little ham-handed] attempts to maintain a sense of workplace decency/propriety in case a company agent has to view the user ID on their CRT or whatever in the course of routine business.
This sounds like a pretty standard response from any big corporation these days, not just nasty old Ma Bell. How this managed to offend you is beyond my grasp, of course, unless you omitted or I overlooked some particularly egregious or perjorative phrasing in their form letter. Maybe you could be more specific. After all, they didn't cast any aspersions on you or your Italian (I think Semenzato is an Italian name, if not I apologize) heritage. They just asked you to pick another user ID.
Nonsense. No one's civil rights have been even remotely violated here. Your telephone service remains fundamentally unchanged (you might not like its quality or price, but that's another discussion !)
Again, AT&T is not discriminating against anyone. I suspect that no one but a bunch of telephone enthusiasts will even be interested --- especially once the facts become clear.
Ummmmmmm, no I don't think so.