To answer it I will take care of root words used in your question, CHILLUM, BAD HABIT, SADHU.
Chillum : Chillum is actually a pipe, it is not what you inhale, the content you inhaling matters, if you are taking higher concentration of nicotine in your chillum it will effect your mental health.You can understand through this.
The content used in chillum is not high nicotine, and a chillum has 'Safi', which is water containing cloth, and removes any kind of containment from the air drawn from it, making it least harmful in health cases.
Sadhu : Someone who does not belong to the word, or according to him, the world does not belongs to him. He considers world a 'MAYA', a conclave of mass, between him and god. He doesn't acquire a family, relatives even no colleagues. He is friendly to natures every being, even to the stone.
Bad Habit : This needs more description to understand. Two questions, Is pissing in bathroom a bad habit? Is pissing on neighbors door a bad habit?, you are right second one is. But if I ask, Is Pissing a bad habit? No of course not, until and unless you are doing it at a wrong place.
But these questions doesn't clarify anything. A doing is a bad habit, if it confirms a loss, or ill treatment, or anything bad to other. Here 'Other' needs to be understood, the other make the thing good or bad. Mostly parents would not like there child to have nicotine, why? he will not be able to study nor be able to do work.
So if Sadhu is having a chillum, is he doing any harm to anyone, no, he doesn't have a family, colleagues to shout for him, neither the chillum harms the environment in any way.
In short NO, chillum is not considered as a bad habit for Sadhus.
Furthermore : .The sadhus are not attached to the world so the worldly thing like chillum does not affect their chitta in the way it does to a man who is a chain smoker,that's why if they do it in nishkam(detached) way to relax their mind that's why it is not bad habit. If even a sadhu does the same by involving into or is affected by not getting it by a specific time period, then that is sakama(totally involved) way and it is a bad habit, or we can say sin which creates bad samskars on chitta.