A friend of mine who moved from Communist Russia to the US once explained to me the "tyranny of choice." He explained that, in the US, sometimes we get overwhelmed with many, many options.
To be quite blunt: After choosing my NAS, the act of choosing hard drives is actually harder and somewhat overwhelming. To be quite honest, knowing that I can choose from a narrower set of drives that are guaranteed to work correctly is going to really tip my scale in favor of Synology the next time I'm in the market for a NAS.
Locking non-approved drives does absolutely nothing for your case though.
They can sell "guaranteed to work" drives for people like you who don't want to go through all the picking process, while letting other people the choice to put any drive they want.
To be quite blunt: After choosing my NAS, the act of choosing hard drives is actually harder and somewhat overwhelming. To be quite honest, knowing that I can choose from a narrower set of drives that are guaranteed to work correctly is going to really tip my scale in favor of Synology the next time I'm in the market for a NAS.