There is one argument for Synology doing this: There have been cases where hard drive companies mislead their customers. I personally fell victim to this when Western Digital started selling SMR drives as WD Red, without labelling them as SMR drives. 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 fa…
Which WD drives specifically were misleading customers? It's fine to have 'Synology supported drives' which guarantee compatability, but requiring them is absolute bollocks.
After some time, people started to post about problems with the new WD Red drives. People had troubles restoring failed drives, I had a problem where I think the drives never stopped rewriting sectors (you could hear the hard drives clicking 24/7 even when everything was idle)
Then someone figured out that WD had secretely started selling SMR drives instead of CMR drives. The "updated" models with more cache were much cheaper disks, and they only added more cache to try and cover up the fact that the new disks suffored from catastrophic slowdowns during certain workloads (like rebuilding a NAS volume).
This was a huge controversy. But apparently selling SMR drives is profitable, so WD claims the problem is just that NAS software needs to be made compatible with SMR drives, and all is well. They are still selling SMR drives in their WD Red line.
Edit: Here's a link to one of the many forum threads where people discovered the switch: https://community.synology.com/enu/forum/1/post/127228