I don't have a problem with past event items being in the shop.

Items, even vanity items, created for the sole purpose of being exclusively on the shop can open the floodgates to a plethora of problems though. I can name plenty of MMOs that had cool in game obtainable vanity items, then cash shop ones were introduced, followed by the future in game ones being made at a much slower rate and at a much lower level of quality compared to cash shop ones.