I think you misunderstood the title. Iran did not attack anyone, it’s militants in Syria who launched attacks and where the USA did already strike back.
And if you want to argue that supporting parties in a conflict should allow airstrikes on the supporting country, that argument is a dangerous double-edged sword. As then your argument would open up strikes on western countries who support parties in a foreign conflict as well. E.g. that argument would fit into Russia’s narrative of the war, claiming all the west is fighting against them in Ukraine.