Shooting them down is a pretty reasonable retaliation for invading their airspace.
There is no cease fire and never was. usa never held up to its side of the agreement. usa fled and Iran chose to stop firing because they had done all they needed to to the bases in the area.
Fighting isn't in Iran's interests. They have won as much as they can militarily. All they want is usa to leave them alone. Their best weapon to get that is choking the gcc and usa by not letting them transit the strait.