Tea is classified as drug because it contains a stimulant called caffeine. DO a search on caffeine and you will get all your answers.
Tea became popular in the panth during the Bristh Raj. Like Opium ( pheem), Tea was inported from CHina under the British hence the name Chaa and Chaee. At first only the " high class" SIkhs who freely socialized with the British consumed this drug the same way they would freely eat meat like the British. Eventually this drug ( tea) was imported heavily and it became popular among the uneducated masses.
In all Puratan Rehatnamey, Guru Ji has put a ban on stimulants such as marijuana ( bhang) , opium, and alcohol. These things never were allowed in the Langar. So why is tea which is a drug stimulant allowed? Why is plastic instead of Sarab Loh allowed in langar. Why does meat- eating patit instead of Tyar bar Tyar Rehatvan Gurmukh prepare langar? Sakhis mention Guru Jis langar was sweet but simple and ingredients mainly included: wheat, sugar, butter, and milk. Same holds true for Guru ANgad Dev Jis langar. In puratan times Bhajan ( praise, devotion ) and Bhojan ( eating) went hand in hand. THe food prepared by AMritDHari Gurmukhs was a boost to ones bhagti. Because food made according to Maryada comes from Guru Ji himself. GUru Ji says if a Gursikh does not believe the food he eats comes from Guru Ji then he is no SIkh at all. THis is why we always recite Twav Prasaad before eating.
Nowadays Langar is a complete joke; Guru Ka Langar has become a social buffet. People come to the GUrdwara Sahib to fill their stomachs not their souls. They treat langar hall as a lounge this is why tea and chit chatting is prevalent.