How about a contractual issue? The lease allows smoking - the landlord is kicking them out for doing something the lease says they can do. It's a violation of a contract.
It's kinda like an Internet provider advertising $9.99 a month unlimited usage.
Then, when you are online for 5 or more hours, you get a warning. Three warnings and they disconnect you. This happened to me. Thank God for DSL!
The lease allows smoking, but the right of quiet enjoyment of adjacent tenants means the smoker must keep his smoke and its odor from unreasonably disturbing his fellow tenants. You have a right to play music in an apartment too, but if you play it at a volume or at such times that it unreasonably affects your fellow tenants' right of quiet enjoyment of their apartments, that may be grounds for eviction as well.
Whether the right of quiet enjoyment is unreasonably affected is a question of fact according to a reasonable man standard. In practice, that is what juries do: determine what is unacceptable according to a "reasonable man."
The reasonable man standard tends to change over time as cultural mores change. Forty years ago, a jury would likely have determined that the complaining tenants were being unreasonable. But then, 40 years ago fully half of the adult population of the US smoked. The "reasonable man" was a smoker or was likely to share intimate living or work space with a smoker. The smell of cigarette smoke was ubiquitous and most people became accustomed to it to the point that they didn't really notice it. That is no longer the case.
Today the "reasonable man" is far more likely to be a nonsmoker who is accustomed to breathing air untainted by the odor of cigarette smoke. He notices the smell of cigarette smoke immediately and he tends to find it disagreeable and obnoxious.
That may sound like tyranny by the majority, but from the nonsmoker's point of view it is simply his right not to be forced to have his nose and lungs invaded and assaulted by someone else's odious and obnoxious cigarette habit in the nonsmoker's own living space (for which he is also paying rent).
The evicted tenants likely could have smoked in their own apartment without disturbing their neighbors. But like a tenant playing Black Sabbath at 130 db and rattling the walls and floor above and beside the tenant's apartment, the volume of smoke exceeded the comfort level of adjacent tenants.