But those people would call support when the array couldn’t rebuild. And many of them would blame Synology, and demand warranty replacement of the “defective” device, and generally cost money and stress.

As long as Synology is up front in the requirement and has a return policy for users who buy one and are surprised, I think they’re well within their rights to decide they’re tired of dealing with support costs from misbehaving drives.

As long as they don’t retroactively enforce this policy on older devices I don’t understand the emotionality here. Haven’t you ever found yourself stuck supporting software / systems / etc that customers were trying to cheap out on, making their problems yours?