Legal?? -Landlord sent a letter saying my smoking ciggarettes is bothering other tenants..?

Yes, it is legal. If your activities within the apartment disturb other tenants, they become problematic. Ya know how you can complain to the landlord if your neighbors are blaring their music at all hours of the night? The same goes for smoking. If the quantity of smoke is enough that it is bothering your neighbors, it's too much. Smoke on the balcony, maybe?