It's because Alcohol is a downer, and energy drinks are uppers. Mixed together, they are incredibly detrimental to your heart.
Energy drinks in general are stupid, the small amount of vitamins and herbs and stuff they put in them don't counterbalance the sugar, artificial flavorings or the fact that it is a carbonated beverage. Those three things together are really bad for you. The caffeine, not so much. Caffeine isn't all that bad for you, it's addicting, and it can be bad in high doses if consumed regularly but thats it.
Coffee is similar, personally, i drink too much probably. I drink about a medium espresso a day. The worst thing about the coffee is the calories, fat (depends on the milk etc) and some of the other crap in it. If you drink way too much of it, it will start dissolving your stomach lining. What I drink is no where near that much, but some people drink 5 cups a day for years which can be bad.
It basically comes down to, whatever is inside the stuff you drink doesn't really matter as long as it isn't incredibly bad for you. What IS important, is moderation. Don't drink 10 fucking cans of red bull, I would think thats common sense. If you use moderation then very rarely is there a problem.