I think it's more the principle. CPU firmware upgrades are not supposed be used for things like this, and if it became normal to use them for removing features, it would just lead to people not updating the firmware at all, and that's not a good scenario for anyone.

Physical hardware products shouldn't lose features after launch. If this was a "mistaken" feature which they suggested it was they should have disabled it on future chips.

Judging by the Reddit threads I saw, A LOT of people were upset even though it was clear that they had not idea what the feature actually provided beyond “encryption”. I’d guess that the majority assumed that the change would result in them basically having to “encryption” in affected AMD devices any more in some vague general sense.