Because all Catholics must believe in Filioque. Its only optional to put it in your creed...
"Your creed"? I thought the RC's also accepted Nicaea and Constantinople. Or are you admitting the universal Church's creed is not your own? Because that's OK...being a non-Christian is not an insurmountable obstacle, it can be remedied if you want.
He was referring to each sui iuris (Melkite Catholic, Coptic Catholic, Maronite Catholic, etc) Churches individual variations of the Creed. And before anyone starts saying that no variation in the Creed is allowed, here is the Armenian variation:
We believe in one God, the Father Almighty, the maker of heaven and earth, of things visible and invisible.
And in one Lord Jesus Christ, the Son of God, the begotten of God the Father, the Only-begotten, that is of the essence of the Father.
God of God, Light of Light, true God of true God, begotten and not made; of the very same nature of the Father, by Whom all things came into being, in heaven and on earth, visible and invisible.
Who for us humanity and for our salvation came down from heaven, was incarnate, was made human, was born perfectly of the holy virgin Mary by the Holy Spirit.
By whom He took body, soul, and mind, and everything that is in man, truly and not in semblance.
He suffered, was crucified, was buried, rose again on the third day, ascended into heaven with the same body, [and] sat at the right hand of the Father.
He is to come with the same body and with the glory of the Father, to judge the living and the dead; of His kingdom there is no end.
We believe in the Holy Spirit, in the uncreated and the perfect; Who spoke through the Law, prophets, and Gospels; Who came down upon the Jordan, preached through the apostles, and lived in the saints.
We believe also in only One, Universal, Apostolic, and [Holy] Church; in one baptism in repentance, for the remission, and forgiveness of sins; and in the resurrection of the dead, in the everlasting judgement of souls and bodies, and the Kingdom of Heaven and in the everlasting life.