The complaints were led by the UK organization, Protect the Pope. Here is an explanation from their website:
The Pret A Manger’s CEO should know better than to use the name of Our Lady, the Mother of God, to sell a snack food. It’s cheap, demeans the name of the Virgin Mary, and offends Catholics who hold Our Lady in the highest regard. It’s irrelevant what the CEO’s employees tell him in the office about the appropriateness of the name, it’s what Catholics think that counts. What will practicing Catholics first think of when they see the name ‘Virgin Mary crisps’? Not a well known cocktail for sure.
There are 53 references to the title ‘Virgin Mary’ in the Catechism of the Catholic Church’, including the following:
The eyes of faith can discover in the context of the whole of Revelation the mysterious reasons why God in his saving plan wanted his Son to be born of a virgin. These reasons touch both on the person of Christ and his redemptive mission, and on the welcome Mary gave that mission on behalf of all men. ‘Mary’s virginity manifests God’s absolute initiative in the Incarnation. Jesus has only God as Father. “He was never estranged from the Father because of the human nature which he assumed. . . He is naturally Son of the Father as to his divinity and naturally son of his mother as to his humanity, but properly Son of the Father in both natures.” (CCC 53-54).
Or how about this from St. Augustine:
Mary “remained a virgin in conceiving her Son, a virgin in giving birth to him, a virgin in carrying him, a virgin in nursing him at her breast, always a virgin” (St. Augustine, Serm. 186, 1: PL 38, 999): with her whole being she is “the handmaid of the Lord” (Lk 1:38).
And Pret A Manger wonders why Catholics feel insulted and offended by this demeaning use of the Virgin Mary’s name?