YAAAY! Many people have taken to blocking all Google Groups posters on Usenet, it's quite literally THE only remaining spam vector there.
the imessage thing is the same as google groups, or if google ran an open SMTP relay, it's gonna result in a ton of spam and people don't care as long as THEY get a personal win.
iMessage does not have open groups for random people to join and participate on.
All the spam control can be done on the client side (e.g. message requests handled entirely separately rather than just landing in your messaging apps front page).
Have yall used other messaging apps? You know this is a solved problem in them? Facebook messenger, signal, telegram (only with premium unfortunately).
In the Usenet case here, Google has the power to disable the spam vectors. They simply don’t give a shit.