1. Any change introduced into a product has a cost outside of dev. In this case, there is a QA hit that I would think is non-trivial. Additionally, new UI had to be created, which presumably involves the art team as well (dev UI is typically... well, let's just call it "functional"). Plus there was presumably PM time spend spec'ing out the behavior. Suddenly your small cost is getting bigger, no?
2. Any change introduced into the product has a regression risk. Even a "simple" addition to the product can have a bug tail that is two or three weeks long. And now your dev is spending time fixing bugs instead of writing new code, and your test time is spending time doing buddy tests and regressing bugs instead of writing new automation. Suddenly your small cost is getting bigger, no?
3. Time spent here represents an opportunity cost. That is to say, the time spent implementing (testing, specc'ing, regressing, developing art for) this feature is time that could have been spent on another feature, or time spent fixing bugs. I will grant that developers are not 100% fungible across a product and so maybe this was the most efficient use of resources, but it seems unlikely.
4. Perception matters. Judging by the reaction from the community, implementing this feature has been fairly controversial. It may be that the amount of anger created by this feature is higher than the amount of joy. I'm unsure, and it'd be tough to gauge that without a lot of research, but the possibility is there. There is a chance that it would have been better overall to not add this feature at all, and not have the developer do ANYTHING. Counter-intuitive, but true. Sometimes the best thing to do is to do nothing.