The way those things tend to be done is that arms shipments to the enemy are attacked when in the territory of the enemy or, at most, international waters.

The reason is pretty simple: it avoids escalating the war beyond control by getting more nations fully involved in the actual fight which would then pull in even more nations with a similar rationale that they support their enemy by sending them weapons.

If it is valid for Ukraine to attack Iran directly because it sells arms to their enemy, then it is equally valid for Russia to attack pretty much every European nation because they sell and even give arms to their enemy.

Activelly going to war with those who supporters one's enemy merelly with weapons is a Pandora's Box that's best left closed.

PS: Also keep in mind that Iran isn't quite breaking this - they're attacking American bases from which attacks on Iran are being launched which happen to be in the territory of a 3rd nation, the equivalent to Ukraine attacking Russian bases in Belarous used to attack Ukraine, which is perfectly valid (otherwise countries could just attack other countries with no consequences as long as they did it from the territory of a 3rd country) - they're not attacking countries which merely sold weapons to America.