"I like action movies" "Here's Megashark vs 747. You like action movies, so you like this"

How is any given subreddit any different? Being into fitness does not mean you share an identical interest/taste/discernment with everyone else who is into fitness. Further social news sites are subject to the tyranny of the minority -- aka the lowest common denominator. When people are more likely to express approval than disapproval, a small percentage of users with a common interest will tend to dominate.