You're right, JN - it's all about money.
It's about the money required to clean the air conditioning ducts, the outflow valves, the innards of the air conditioning controls...
It's about the money the insurance companies want...
It's about the money the passengers have to pay for their tickets, and how much money your competition want...
It's about how much money your non-smoking passengers will take to your competition when they find out that people will be smoking on your flight...
It's about the cost of making the modifications you suggest to make it remotely tolerable for non-snokers aboard your aircraft...
The bin extinguisher - yes. Perhaps it can cope, bravely does it's duty and puts out the fire. What then? Close a toilet for the duration of the flight. Inconvenience everyone else because some smoker is too self-centred to obey the rules.
No accident due to legal smoking? Perhaps not. However, the anti-smoking brigade, insurance companies and airlines all discovered that, provided passengers behaved themselves, it costs a lot less to operate aircraft all non-smoking than allowing it. The dangers and accidents then followed because people are incapable of restraining themselves, or planning ahead by bringing nicotine gum or patches or inhalers, and were that selfish that they'd endanger an aircraft full of people for a quick drag.
I hasten to point out that I am a smoker. However, I am a smoker who can contain himself, or plan ahead. Why can others not do so?