Exactly. For previous events, you could easily farm more than one hundred of the event currency, which quickly led to a deflation of the value.
For the Chevin event, they tried to increase the value again by adding in the red Sparkle Powder as a money drain, but since most people couldn't care less about social items, they were not in high demand.
The Gree event ensures that you can only get 14+7 gray helices per character per week. That way, people with more characters or more time do not have an advantage over casual players.
Previous events? You do realize that Gree event is the 1st recurring event they did. Bounty Weekly and Life Day came after the Gree Event was placed in the game. Both you can buy event currency. The Gree Event is over year old.