The compatibility list
Having a compatibility list is perfectly understandable and acceptable, forcing customers to choose one type of drive is not.
Synology is not in the business of making drives. It can most definitely say which drives are preferable, but it is not credible when it states that only "its" drives are acceptable.
A compatibility list is clear - you purchase an element of the list and you can expect proper performance and reliability. You purchase outside the list and you're on your own. We know the rules.
Synology has a deal with Toshiba, fine. Point us to that fact and wax lyrical about how that specific range of Toshiba drives are the best, fine. Don't put your name on it and pretend that they're the only ones that work.
You're not a disk maker. Stop pretending you are.