There are plenty of providers that when you BYOIP, they will broadcast out of their ASN, I know Azure does, Google appears to, no clue on AWS. Plenty of colo providers including $LastCompanyProvider will fold your IP block under their ASN as well. That's how it worked at last job.

Sure, Iran government may just decide to block that specific ASN but if it's they want to remain somewhat on the internet, they are stuck with "Smack entire broad ASNs and lose large chucks of internet" or "Block specific IP spaces."