Because the raspberries are cooked.
As soon as you cook, mash, purée, juice, blitz or do anything to fruit except chop it and eat it, it becomes synned.
To be fair the Syns aren't that bad, generally 100g of most berries is only 1 syn, and from what I've heard about this recipe it's well worth that 1 syn
Still haven't made it myself yet, that'll be a weekend job!