The Belgrano was outside the Exclusion Zone and, at the time of the attack, was sailing away from the Falklands.
The counter argument was that it it was armed with Exocet missiles and, according to intelligence, under orders to rendezvous with other ships to attack the British task force.
If we take the Iris Dena then it was highly likely to be returning to Iran and would therefore have encountered US vessels at the Eastern end of the Gulf, unless anyone would like to suggest that it was more likely to be going on a cruise to the Maldives.
The US Navy had absolutely no way of telling whether the vessel was heavily armed or not.
Lastly and getting to the nub of the question, Iran clearly doesn't follow international rules with state sponsored terrorism in the region, 20 attempted terrorist plots in the UK, kidnapping of British sailors and launching unprovoked missile attacks at numerous countries in the region.
So why do you insist on different standards for the US in a conflict with Iran?