The OP has a point.
While Runes of Magic calls itself Free-to-Play, they are only Free-to-Play in the most lenient sense of the term.
Outside of the overworld (and sometimes not even there) F2P players have to move mountains to even come close to anyone who buys diamonds. Part of RoM's success plan is to make sure its players (F2P or not) are dependent on the Item Shop, and by extension, diamonds. Phirius tokens are a limited resource (unless you buy diamonds for reset tickets, of course) and the items that can be bought with these tokens are few and expensive.
While most 'special' sales (limited time items) ARE indeed offered in the phirius token shop during the promotion, Gameforge may just be desperate enough to play the scarcity card and force phirius token using players to buy diamonds in order to get what they want.
My speculation is that Gameforge is indeed desperate. It would certainly explain forcing players who want +16 gear to buy diamonds and the rather frequent 100%, 120%, and 140% diamond sales.
[Removed Explicit Comment]
Edit : Please keep your comments clean ~Majik