Any diet that prohibits you from eating an apple is ridiculous in my opinion.
New diet fads come and go, but the bottom line to weight loss is simple math calories in calories out. Keto isn't magic.
When you eliminate carbs from your diet you are just restricting your calorie intake in a round about way. Most people find it difficult to over eat with proteins and fats. Most overeating is carb based. Chips cookies, soda, and the like are the main culprits.
I imagine if you tracked the caloric intake of the people who have had great success with a Keto diet, you'd see a significant caloric reduction with the Keto diet which led to the weight loss.
I get what you're saying about fad diets and I'm sure we'd agree there. However, your theory about the simple math of calories in calories out just doesn't agree with science, or at least the studies I've read.
I'm not sure if a strict keto diet would prohibit an apple or not (probably depends on what else you're eating too), but I definitely wouldn't think twice about eating an apple. As long as I didn't plan on sitting still for a long period of time afterward. Apple juice however is an entirely different story. You're body has no idea what to do with a blast of that much sugar with no fiber.