If this happened at one of my parties (I generally host and provide food and some drinks. People usually bring some extra beer and wine) I might think it's a tad odd, but really I don't think I would notice. People show up, I say Hi, I offer them a drink, direct them to the food and that's about it. Drinks are generally self serve, but I get them glasses or perhaps pour them a first drink (more likely with early guests).
So while the behaviour is a little odd, I would have a hard time noticing or caring enough to complain about it. (If they parked in the wrong place I would ask them to move. I would never assume people know that they can't park in my driveway.)
This strikes me as an incomplete story with some background I don't know about.
As an aside, most one year olds birthday parties I've been invited to have had alcohol, no gifts and often the guest of honour naps through most of it. They are excuses to get together. I would find drinking inappropriate at a kids birthday party (ages 8-18) but not at a younger kids party. Somehow it seems more inappropriate as the kid gets older.