Well, AIS won't register more than one iris under any given name. So, the person in your example who introduced an iris without registering it would have to either
a) register it and sell it under a different name (and presumably explain to their customers why its name was changed)
or
b) keep selling it under the original name as an unregistered iris.
I didn't say it is a good idea, merely that it can, and does, happen. No plant society, AIS included, can compel anyone to do anything. Breeders and sellers follow the rules (or not) because they believe it's in their own interests to do so.
Here's an example:
Tall Bearded Iris (Iris 'Rock Around the Clock') was originally listed in the Stout Gardens catalog under the name 'Madame Butterfly' (you can still see the name in the upper right corner of the picture). However, the registrar rejected that name as being too similar to
Tall Bearded Iris (Iris 'Madam Butterfly'). If Lowell Baumunk and Stout Gardens really wanted to sell it under the name Madame Butterfly, they certainly could have, although AIS wouldn't have registered it under that name. They chose to change the name, presumably because selling unregistered irises is bad for their business.
AIS typically lets you know if your registration applications have been accepted or not in mid-January each year. I believe that's why most commercial growers wait until late January or February to publish their catalogs and update their websites.
