So lots of customers thought they were buying a drive that's perfect for NAS, only to discover that the drives were completely unsuitable and took days to restore, or failed alltogether. Synology had to release updates to their software to deal with the fake NAS drives, and their support was probably not happy to deal with all the angry customers who thought the problem was with Synology, and not Western Digital for selling fake NAS drives.
If you buy a drive from Synology, you know it will work, and won't secretly be a cheaper drive that's sold as NAS compatible even though it is absolutely unsuitable for NAS.