Season's Greetings

What the hell "Season's Greetings" means? I don't get it. Maybe because I'm particularly cranky these days, I always get cranky around the holidays. So I guess I will never really understand the "Season's Greetings".
Today I went to Save On Food to buy some stuff for Christmas, I also bought something for myself, like this figs jam. Well, the cashier, a nice looking woman around her thirties, probably trained to do so, asked me "What do figs taste like? they're sweet, right?" Yes they are, I replied, and then she said "Usually people buy apricot or strawberry jams... but I guess all jams taste the same..." They taste like jam! I thought, then I said that I like the figs more, meanwhile I was thinking "You would taste like any other jam, why are we having this conversation?" Why are people trained to be nice, there is nothing wrong to me in just being kind and professional. There I am, cranky again.