Why does the "drunken, jubilant mob" in Bielietz believe it has been liberated?

All of the people wanted to be seen as German Poles or at least as sympathizers to the invading Germans so they might be safe.

There has always been a specific distinction between being a Jew and being a Pole. The Jews had lived in Poland for 600 years, but they are still seen as outsiders. They have not attained equality socially or legally, and the distinction always is made between being a Jew or a Pole. To the Poles there is no such thing as a Polish Jew; Poles are Polish and Catholic and Jews are alien. Even though Jews serve in the Polish army and in government, most Poles do not view Jews as Poles.

On the first day of occupation, some of the Jewish men are arrested, others are taken from the street and placed inside a synagogue, which is set afire. The Germans have been greeted as liberators by the Weissmann’s Polish neighbors, furnished with welcomes, and liquor, and flowers. The Jews see this as betrayal by their neighbors.