Is this even against the rules?
Rules? I don't think there are any rules that extend beyond the platform and control interfaces, in other words, I doubt there are any rules that refer to consumption of anything.
Where's the line to be drawn? Some people perform better on certain chemicals, others perform worse. Some have actual diagnosed diseases where that amphetamine or other stimulant is actually going to slow them down.
In some ways, one could argue certain chemicals will give an unfair advantage - as while they are technically video games, they are also contests of mental strength, hand-eye coordination, reaction time, and game-specific strategy and planning sometimes many moves ahead.
Do board game competitions have rules prohibiting chemicals/drugs? I think that is where e-sports shares the most in terms of competition format. E-sports shouldn't have anti-doping rules at the level of athletic and endurance sports, they should have similar rules to board game and trading card competitions.
What are the rules of competitive chess matches? Can they take stimulants or prescribed nootropics?
Nootropics like modafinil could have a boost in this arena of competition, so I am extremely curious how our society will react to the nootropic movement alongside the e-sport movement.