Earlier quoted context omitted.
While caffeine is technically a drug, it's just very different than alcohol and other drugs. People can go to work, spend time with family, and even make important life decisions without caffeine getting in the way. The same is not true for alcohol, cocaine, meth, etc. (Nicotine is similar, I imagine, though I've never actually used it)
I dunno. I think it's whatever society will tolerate. It used to be pretty common in America to drink on the job. Plenty of folks worked/work and do cocaine. Heck, Freud was a huge cocaine addict, and he's still widely celebrated for his stim-induced work!
These isolated cases without incident don't really matter in the context of society-wide taboo, however.
Coffee and cocaine are worlds apart. Most people don't have jobs that would work with a coke addiction. Freud was exceptional and I think he did most of his influential work not under the influence anyway.