I have never received a single spam message through iMessage, but regardless. If they have to keep buying new phones all the time as they get blacklisted, in order to keep spamming, the economics of spamming changes.
That does raise the bar a little bit but it's probably mitigated by the combination of targeting higher spending users and the additional trust iMessage provides.
The difference between being able to send unlimited messages for free, and having to get a new phone every hundred or so texts is pretty large. Free vs 1 dollar per spam text.
I did recieve some spam messages there, so Apple's claims about muh security etc. already is bogus