Yes but ecumenism is the official teaching of the Catholic Church since Vatican II,a fact that has been ratified time and time again by the modern Popes themselves through encyclicals (Ut Unum Sint) and actions (praying in synagogues, mosques; assisi, etc.). It is true that the earliest Christians forbade and even anathamatized those who would pray with those not of the faith but that has been changed since Vatican II. If one is to accept the authority of Vatican II, the modern Popes and even the 1983 code of Canon Law one cannot really object to ecumenism and not be at odds with the official teachings of what Catholics believe to be a valid Ecumenical Council not unlike that of Nicaea, Ephesus, Chalcedon etc. Vatican II and the modern Popes have actually made things like this ecumenical prayer service with Cardinal O'Malley not even possible but acceptable and par for the course. In fact Vatican II even states that priests need an "ecumenical formation" as part of seminary education. That this new orientation is in flagrant violation of millenia of Church teaching is undeniable and yet it happens anyway.
As for this recent "event" all I can say is that it is probably more about a community trying to be in solidarity in the midst of a tragedy than anything else. Sentimentalism and as someone else said "cermonial deism" reign in events like the one mentioned. That being said it is still in bad taste I think. It would have been better for Cardinal O'Malley to have Solemn Vespers and pray for the dead and make it a somber affair but unfortunately the brand of Catholicism in the mainstream is the happy clappy, sentimental Communion in the hand "God is love" "everyone goes to Heaven" kind. Traditionalism is a tiny minority and a persecuted one at that. As a good traditional priest once said "there is a caste system in the Church and we (traditionalists) are the Untouchables."