I think PTPD2 covered it nicely... But I'll add a few thoughts...
Based on what you've described, the ticket is well justified...
Left turns are regulated under the following Iowa Code section:
321.320 Left turns — yielding.
The driver of a vehicle intending to turn to the left within an intersection or into an alley, private road, or driveway shall yield the right-of-way to all vehicles approaching from the opposite direction which are within the intersection or so close thereto as to constitute an immediate hazard, then said driver, having so yielded and having given a signal when and as required by this chapter, may make such left turn.
[S13, §1571-m18; C24, 27, 31, 35, §5035; C39, §5026.02; C46, 50, 54, 58, 62, 66, 71, 73, 75, 77, 79, 81, §321.320]
For applicable scheduled fine, see §805.8A, subsection 7
Additional penalties for violations causing serious injury or death, see §321.482A
In theory, and from a stand point of legal obligation and compliance with the rules of the road, the officer is correct...
That really is not an accurate interpretation of what the officer stated; At least that is not the lesson that is to be learned here. I don't know of anyone who would intentionally slam into someone when their rear end is in front of you. I think it would be a spontaneous natural reaction to try and avoid a collision...
But since you put it that way, if you do happen to slam someone when their rear end is crossing in front of you -when you have the right of way- then you are not legally liable for any damages that result.