Why is coffee provided during big chess events when it contains a banned substance? Why is coffee provided for free during big chess events like the Olympiad despite the fact that it contains caffeine, which is a banned substance according to the World Anti Doping Agency (WADA)?
According to FIDE's Anti-Doping Policy:


  The 2018 WADA Prohibited List and Monitoring Program can be found at:
  http://list.wada-ama.org/
  The most relevant banned substances for chess are:   
  
  • Amphetamines – e.g. Adderall, Ritalin
  • Ephedrine and Methylephedrine – Prohibited by WADA when its
  concentration in urine is greater than 10 micrograms per milliliter
  • Pseudoephedrine is prohibited when its concentration in urine is greater than
  150 micrograms per milliliter
  • Modafinil
  
  Substances not present on the Prohibited List but represented in the Monitoring
  Program:   
  
  • Caffeine – Included in WADA Monitoring Program and relevant for incompetition testing only. Any urine test reading of less than 12 micrograms per milliliter poses no problem.
  • Codeine – A common ingredient in, for example, preparations used to treat coughs and stomach upsets. Any dosage is highly unlikely to be significant when taken in normal therapeutic quantities.    


Note that caffeine is not currently banned. It was banned during the period 1984 - 2004 but was lifted. I'm guessing that sponsorship via advertising from Coca Cola may have been an important factor.