There are two principles of Hare Krishna philosophy which are expressed practically by the practice of vegetarianism. These two principles are:
1) Non-violence
2) Service to God (Krishna)
Non-violence means not to stop the progressive life (materially or spiritually) of any living being. According to the laws of karma and reincarnation if an animal is killed before its allotted time in that particular body has ended then it has to take birth again in the same type of body in order to complete its remaining days and be promoted upwards to the next species. Thus its evolution upwards through the different species of life is checked. Therefore, the killing of animals simply to satisfy the demands of the palate is an act of both material and spiritual violence.
As far as service to God is concerned in Bhagavad-gita it is said: “If one offers Me (Krishna) with love and devotion a leaf, a flower, a fruit or water, I will accept it.” (Bhagavad-gita 9.26)
Before eating anything devotees of Krishna perform a ceremony whereby they offer their food to God. This religious performance sanctifies the food and frees the person eating it from the karma involved in the collection and preparation of the ingredients.
Because in the Bhagavad-gita Krishna says that He will only accept food if it is vegetarian then Hare Krishnas only eat vegetarian food. This they do as an act of service to please God (Krishna).