Small things that break you suspension of disbelief or immersion

[Keltest]

It isn't always obvious if the king and the taster have entered an area with antimagic or magic nullifying properties otherwise. Retaining the taster adds a layer of redundancy in case the belt fails either because of antimagic or because of the invention of some kind of poison that otherwise bypasses normal poison immunities.

Also, just because you aren't dying from it doesn't necessarily mean you wouldn't be able to tell it was poisoned.