Whatshaiches, I looked up the Igros Moshe in question, and thanks a lot for it. You basically got it. He says assur because:
1. damages the body ( lechora same is true for cigarettes)
2. damages the mind and prevents proper learning and doing mitzvos (lechora same is true for drinking)
3. Causes “the munchies” aka excessive taavah to eat and drink more than the body needs. this gluttony is a key part of a Ben sorer umoreh (would he also apply this to the davar mechuar of “all-you-can-eat” gluttony?)
4. It could lead to theft to fulfill taavos, ie get the next “hit”.
5. Causes excessive pain to parents, and violates kibbud av ve’eim (lechora also applies to drinking and smoking)
6. Kedoshim Tihyu of Ramban in the beginning of parshas kedoshim- a mitzvas assei not to overindulge in pleasures, not to drink too much alcohol, not to overeat, etc. (lechora this mitzvas assei would assur all-you-can-eat but I’ve never seen it implemented)
7. Gateway to other aveiros.