They're not irregular plurals. They're simply uncountable nouns and don't form plurals under normal circumstances.
And under normal circumstances you’d be right (like, maybe gossiping around a knitting circle). But team identities create something other than normal circumstances, for both organizations and fans.
I’ve heard people here in Utah say “he’s a Jazz” throughout my life. If there’s more than one player who is a “Jazz”, then it becomes a plural under the standard creaed by all the other mascot plurals.
I’m sure the common word usage is similar in Oklahoma and Florida.
That type of usage makes for a set of irregular plurals within the context being discussed on this forum.
I’m completely game if you’d like to flesh out your argument to the contrary, though. But you’re going to have to do better than relying on “normal circumstances”, which probably don’t even exist anyway when you realize that the definitions/qualifications of words for dictionaries like the OED and Webster have been relying on descriptive understandings just as much as prescriptive for the last fifty years.
I don’t think there’s a “queen’s english” type of authority that can even be pointed to with regards to the NBA. I mean, Mitchell handled Shaq with an “Aight”. So it’s just as likely to be Patrick Beverley as Adam Silver.
