So what is Zapatero doing there, besides giving bible readings - I thought he is a socialist.

Well, he's delivering a jab to the English-only movement in the US:

Gracias a los Senadores Klobuchar e Isakson, y permítanme que les hable en castellano, en la lengua en la que por primera vez se rezó al Dios del Evangelio en esta tierra.

My thanks to Senators Klobuchar and Isakson, and allow me to speak to you in Spanish, the language in which for the first time the God of the Gospels was prayed to from this land

(The Spanish were in North America before the English)

His choses bible passage was

Permítanme que les lea un pasaje de la Biblia, del capítulo 24 del Deuteronomio: "No explotarás al jornalero pobre y necesitado, ya sea uno de tus compatriotas, o un extranjero que vive en alguna de las ciudades de tu país. Págale su jornal ese mismo día, antes que se ponga el sol, porque está necesitado, y su vida depende de su jornal".

Bible Gateway: Deuteronomy 24:14-15 (New International Version)

14 Do not take advantage of a hired man who is poor and needy, whether he is a brother Israelite or an alien living in one of your towns. 15 Pay him his wages each day before sunset, because he is poor and is counting on it. Otherwise he may cry to the LORD against you, and you will be guilty of sin.

The US not a secular country: it is a very religious country. When in Rome, do as the Romans. While it is debatable whether ZP should have been at that event in the first place, given that he did go he's probably used the opportunity to make a very good socialistic speech.

En un viejo país ineficiente, algo así como España entre dos guerras civiles, poseer una casa y poca hacienda y memoria ninguna. -- Gil de Biedma