Part of the issue is they communicated poorly. They have done either a credit or earn option for every other pack/character/weapon with exception of the black steel ones, which, rightly or wrongly, led to expectations of a similar format.
With these, there were hints, rumor, etc, that the “glowies” were being added, which got people excited. Then it was basically confirmed for this weekend, which added to the anticipation. With the announcement that it’s real money only (and probably overpriced) the excitement turned to disappointment.
If they had just said from the beginning that the lambent were coming as a cash only option, there would likely have been less “outrage”. They were upfront when it came to the Rise of Raam, and with the supporter packs; and while there were some complaints, most people didn’t take issue.