Go to a tea shop and order some Dragonwell, and drink it. Then have a bowl of matcha. You will feel the difference, and it can be a rather alarming one.
Edit: Why that's a big distinction: Your body typically flushes water soluble chemicals fairly easily. It has a harder time with the ones that aren't water soluble, so they tend to accumulate over time. So capsule of tea leaves is probably not the same thing as a cup of tea, from a pharmacological perspective, and theres good basis to assume that it has a lot more ability to hurt you. The same goes for pill version of traditional medicinal teas (echinacea, kava kava, etc.)
Funny that you picked that as an example: https://en.wikipedia.org/wiki/Cinnamon#Toxicity
Same thing: tasty, used for ages, but problematic in large quantities.
Think about the amount of green tea (the plant) that you actually consume in a cup of tea. Even if it's loose tea and you've done a sloppy job, perhaps a few granules.
Now imagine popping a couple of capsules packed with the stuff every day. All of a sudden your intake of something has spiked a few hundred times. It could be cinnamon and you'd have health consequences.