Because presumably you're a damn adult, not a 5-year old with undeveloped taste buds. It's one thing to put ketchup on a burger (although I'd argue that thousand island dressing is far better along with any kind of a spicy aioli is far better) along with mustard.
Being something of a steak aficionado, if you're going to order a dry-aged strip steak (which is what we're talking about in the case of the American President), you want to taste the earthiness, the intense beef flavour, and the minerality of the steak. Pouring ketchup defeats that purpose.
You also don't want to turn the damn thing into a hockey puck. A steak should be served medium rare...medium if you must. Well done? You could serve that idiot shoe leather or a car tire and I doubt he'd know the difference. If it's well done, then clearly he has no sense of taste and has the brilliant idea of ruining a $52 steak by pouring some high fructose corn syrup on it.
What's sad is that somehow, this will become a thing because the lemmings will think "let's support the idiot by pouring ketchup on a really good steak" which follows the idea of 'hey, if Skeeter jumps off a bridge we should do the same'.
If I'm grilling a skirt steak I'll make a chimmichuri sauce (parsley, cilantro, olive oil, spice, garlic) on it. But for a tomahawk rib-eye or any Prime-grade cut of beef, just serve it medium rare and let the quality of the beef do the talking.