The Seiks receive Proselytes of almost every Cast, a point in which they differ most materially from the Hindoos. To initiate Mohammedans into their mysteries, they prepare a Dish of Hogs legs, which the Converts are obliged to partake of, previous to admission??????..They are not prohibited the use of Animal food of any kind, excepting Beef, which they are rigidly scrupulous in abstaining from.

The seiks are remarkably fond of the flesh of the jungle hog, which they kill in chase: this food is allowable by their law. They likewise eat of mutton and fish; but these being unlawful the Brahmins will not partake, leaving those who chose to transgress their institutes to answer for themselves.

Now become a Singh, he is a heterodox, and distinct from the Hindoos by whom he is considered an apostate. He is not restricted in his diet, but is allowed, by the tenets of his new religion, to devour whatever food his appetite may prompt, excepting beef.

The reason why meat is not served at langar in the Gurdwaras is because langar is supposed to be a symbol of equality of mankind where all people no matter what race, religion or caste can eat together in the atmosphere of brotherhood. Hindu, Sikh, Muslim, it does not matter who they are. Different religions have different dietary restrictions. Hindus cannot eat cow, muslims cannot eat pork and will only eat halal meat. Jews will only eat kosher meat, others cannot eat fish or eggs. But in a gurdwara langar, it does not matter what their dietary taboos or religious beliefs are, the food is designed so that all can eat together and no one will be offended or not be able to partake of the meal.

What is Jhatka Meat and Why?

Jhatka meat is meat in which the animal has been killed quickly without suffering or religious ritual.

Sikhism, A Complete Introduction, Dr. H.S.Singha & Satwant Kaur, Hemkunt Press
We must give the rationale behind prescribing jhatka meat as the approved food for the Sikhs. According to the ancient Aryan Hindu tradition, only such meat as is obtained from an animal which is killed with one stroke of the weapon causing instantaneous death is fit for human consumption. However, with the coming of Islam into India and the Muslim political hegemony, it became a state policy not to permit slaughter of animals for food, in any other manner, except as laid down in the Quran - the kosher meat prepared by slowly severing the main blood artery of the throat of the animal while reciting verses from the Quran. It is done to make slaughter a sacrifice to God and to expiate the sins of the slaughter. Guru Gobind Singh took a rather serious view of this aspect of the whole matter. He, therefore, while permitting flesh to be taken as food repudiated the whole theory of this expiatory sacrifice and the right of ruling Muslims to impose iton the non-Muslims. Accordingly, he made jhatka meat obligatory for those Sikhs who may be interested in taking meat as a part of their food.

Sikhs and Sikhism, Dr. I.J.Singh, Manohar Publishers.
And one semitic practice clearly rejected in the Sikh code of conduct is eating flesh of an animal cooked in ritualistic manner; this would mean kosher and halal meat. The reason again does not lie in religious tenet but in the view that killing an animal with a prayer is not going to enoble the flesh. No ritual, whoever conducts it, is going to do any good either to the animal or to the diner. Let man do what he must to assuage his hunger. If what he gets, he puts to good use and shares with the needy, then it is well used and well spent, otherwise not.

Many person became his disciples. Nanak believed in the Oneness of God and in the way that it is asserted in Muhammadan theology. He also believed in transmigration of souls. Holding wine and pork to be unlawful, he had [himself] abandoned eating meat. He decreed avoidance of causing harm to animals. It was after his time that meat-eating spread amongst his followers. Arjan Mal, who was on of his lineal succesors, found this to be evil. He prohibited people from eating meat, saying "This is not in accordance with Nanak?s wishes". Later, Hargobind, son of Arjan Mal, ate meat and took to hunting. Most of their [the Gurus] followers adopted his practice.