I agree with KirkS on the 'should be able to leave whatever feedback you wish' idea.
Perhaps with that check box eBay is trying to prevent the angry sudden response and let one cool down a titch and then respond more calmly. That feedback is not removable once left. (Kirk, you sound like you were calmed down and rational.)
The customer is not always right IMHO but it is good business practice (I think) for the seller to be able to take an opportunity to listen to honest feedback and hopefully correct any wrongs or misunderstandings or miscommunications.
That was a no-no to ship an item that, in the mail system, would be fragile and likely to be damaged. He could have wrapped it in something first and then put it in a heavier envelope. Protected it somehow. Not just to protect the item but to prevent future non-positive feedback and bad feelings.
I sold a thinned stamp once and didn't notice it (I was ill and tired for a week) and the person left positive feedback, but YELLED (with capitals) that the stamp was severely thinned. I was surprised but looking at the back scan of the stamp I did notice the thin when examined closely. I was embarrassed and offered a full refund plus plus. However, I think the fellow was so angry himself to have me 'rip him off' that he must have destroyed the stamp first and left feedback second. He never did respond. I asked twice if he wanted a refund. But, I feel, his feedback for me was deserved. I messed up. Hopefully I learned better. You make a mess, you clean it up.
As laswabbie says about the DSR stars (detailed sales ratings), the fellow left me all 1's and ruined my record for months. Not really deserved, in hindsight knowing the whole story, but it happened.
I guess my long-winded point is that mistakes can happen and it would be nice if everyone could realize that and be able to forgive and then communicate and resolve the matter to everyone's reasonable satisfaction. But, I realize, the above is a lot of shoulds and ifs and you would be lucky to have that happen with every buy or sell.
But to maintain a good business and customers over time, your reputation is everything. And there is why the customer thinks he is always right. But on eBay the seller can block a buyer from buying from him. Preventative medicine.
I don't think you are wrong Kirk. Everyone is different in their need and skills at confrontation and sales and talking to people or emailing.
I think the seller could (and should in a perfect world) learn from his mistake no matter how the feedback was left. It wouldn't kill him, not one feedback, and it is up to him to choose to take advantage of the opportunity to learn something. A negative or neutral or a positive. He could learn either way. And it is nice to put a warning flag up for new prospective customers. He could respond to the feedback, explaining his side, and that would tell customers something also, how he responded tells something about what to expect.
I guess in this world you can only do the best you can do.
I should be a politician, I think I took both sides at once.