Excuse me, do you think it is Farsights decision or not?
I have seen an image of the document and it doesn't say that the developer/publisher have to participate.
Right. They don't have to, that's pretty obvious. Never said they do.
Farsight either didn't want to participate in the sale or they didn't manage to apply for the sale in time. I'd say the latter is less likely since some of the games that appeared on Steam days before the sale got a discount anyhow. But it's not impossible, their communication with Valve is horrible, they had problems with appearing on Steam store properly. I'm pretty sure we won't see TPA taking part in the winter sale.