[Pseudoscience-is-malarkey (OP)]

It is said that it might be the most important job on earth. It is a job that takes 100% focus and calm. How can an ATC focus if they're having a nicotine withdrawal?

[Bored chemist]

That's a reason why they should not smoke.
Non smokers don't get withdrawal symptoms.

but some of them do smoke.
So, not letting them would provoke the withdrawal symptoms you are keen to avoid.

[paul cotter]

Nicotine does enhance cognition slightly but the effects are short lived. Several medications that are agonists at the nicotinic acetylcholine receptors are used in dementia treatment with a small degree of efficacy. I have not smoked in nearly 40years but I still enjoy nicotine( gum ), especially if I have a problem to solve requiring intense concentration.

[alancalverd]

Depends on the environment. Some ATCOs work alone on small airfields so whatever keeps them alert and happy is a Good Thing, but if you have anyone else in the tower it becomes an Enclosed Workspace and smoking is generally banned from EWs in the UK*.

Time was when everything was done on paper and wood blocks, and the tower was fairly robust, but having seen the pandemonium caused by an ATCO spilling a glass of water on a modern computer system, I'd probably put a ban on coffee (which definitely has an effect on performance) anywhere near the operational desk. But the FAA actually requires pilots intending to fly for more than 4 hours to carry tea or coffee if they usually drink it, because  caffeine withdrawal really does slow you down.

*insane recollection. I was involved in tracking some radionuclides that had been in a warehouse that caught fire. We had a cherrypicker to allow us to inspect the tangled mess of high racking in this utter wreck of "cargo, various, oxidised" and twisted metal. One day the driver lowered me to the ground and apologised as he wanted to go "outside" for a smoke: even though 50% of the roof was missing and parts of the floor were still steaming from the fire hoses,  it still classed as an Enclosed Workspace and H&S took a very dim view of any transgressions!

[Bored chemist]

If I was looking for something nasty in the remains of a warehouse fire, I think my respirator would have made smoking difficult.

[paul cotter]

Following on what Alan said, I have seen the damage when a cup of coffee went into a broadcast audio desk. This was 40 years ago and being an analogue desk with simple circuitry and large print tracks it was easily repaired. A modern digital desk would not be recovered so easily and might expire.