Not how the system works.
It should be, almost every major retailer today gives you back some sort of cash or credit if you purchase something a week prior then it goes on sale.
Even several online games has this feature, so I don’t see why Anet would want to be any different (other than their whole Nexon mentality lately).
I suggest try contacting support and try fighting it.
I brought up this exact same argument, but unfortunately their official response is “all sales are final”.
I, too, spent 1600 gems on New Year’s day that I’d gotten for Christmas. I specifically waited to see whether or not they would be having an after Christmas or New Year’s sale. Since there was no sign of one by the 1st I went ahead and purchased two character slots. Then 3 days later there’s a surprise announcement of a “New Year’s” sale, of course with character slots on sale. Awesome.
I can’t imagine anyone who bought/received gems for Christmas was at all pleased with this sale. Anet certainly lost points with me over it. The sad thing is if they’d reimbursed the difference I probably would have ended up buying more gems, instead that’s probably the last purchase they’ll ever see from me.
I saw no mention of the sale in the launcher until the patch on Jan. 3rd. I received a gem card for Christmas, specifically did NOT spend any gems (despite desperately wanting to) until New Year’s in case they had a sale, and now this.
I’m waiting to see what the next two days hold. So far nothing on sale is anything I purchased, but when I first heard about it I was sorely tempted to petition for a gem refund. At the very least it would be a nice gesture if they reimbursed the difference for anyone who’s made a purchase in the past week for any of the items on sale.