They obviously could start methodically destroying civilian infrastructure to cause a huge humanitarian crisis which might force the Iranian government to collapse. They have clearly chosen not to commit to that approach so far.
These kinds of arguments are obviously vacuous: The US could also "win" any war with a non-nuclear opponent by nuking every major city. But the fact that they don't is not something worth calling restraint.
It’s definitely worth appreciating. The world could have looked very differently and might still fall into that trap.
They did that btw. Is there any railroad bridge still standing in south Iran?