SayIt, you need to read the entire prohibition you posted. It doesn't say all psychoactive substances are banned, only those "which may render him unable to perform such function in a safe and proper manner." Caffeine is one of the more thoroughly studied compounds around, and has been shown to be generally very safe. It increases alertness and energy, but doesn't cause a rebound effect when it wears off. In effect, it actually increases the ability to perfom such functions in a safe and proper manner. Prohibiting the use of caffeine in the US would undoubtedly result in an armed insurrection, and could never be enforced. I suspect it might be the same in Europe.
As for aspartame, or NutraSweet, and any other artificial sweetener, I just can't take them. They all taste bad, and I'll go thirsty before I drink something sweetened with them, even if they were proven absolutely safe.