American Black bears are far and away the most numerous bear in the world by some margin. It is thought that at least 800,000 American black bears still roam the continental north America – though the closely related Asiatic black bear has a far smaller population of about 50,000 spread across south-eastern Asia and is therefore far more endangered (the spectacled bear is also thought to be relatively closely related).
In America, black bears are relatively common sites where they are found.
While both grizzly bears and wolves occasionally predate on black bears, and therefore may well have kept populations lower if they themselves had not been hunted near extinction, black bears are clearly the species that will naturally be most numerous.
While mostly afraid of humans, in many places as humans push into wild habitat the black bears have become conditioned to humans.
In this instance, it appeared during the party, and undaunted by the loud noise and shouting by the guests, it sauntered over to the table, and helped itself to some cupcakes before wandering off back into the wilds.
This is not a particularly rare occurrence in parts of America. Unfortunately as the population continues to grow, these sorts of interactions are likely to increase and may become more dangerous. Of course in many parts of the world, animals of this size have been eradicated, so while there is some negative reactions in the USA, this species has done far better than close relatives.