You didn't answer my question as to why if it is such a bad idea, then folks are willing to do them. The answer is not that they are weak residents without better choices. In my experience, the opposite is true of them.
However, to assist those who are interested in understanding, not just describing this training as being a "joke" "scam" "useless" and "ridiculous" to mention a few recent adjectives used specific not for general pedi or IM but to academic general fellowships, let me explain SOME of the reasons that they might be selected by very smart people who understand the personal and financial aspects of the training and the ultimate career.
Doing this type of fellowship serves as protected time to begin an academic career doing things that are more difficult to get started in as a new attending. In these fellowships, the fellow has 80-90% protected time to do things like take a class or get a degree (usually MPH, but not always), develop curriculum, and set up a range of Qi projects amongst many things. Although there is some aspect of having more training/mentorship time, it is actually the protected time that matters the most. Those who would like to have a career in academic pediatrics (prob true of IM, but I don't know that world) as a primary career goal will want this ramp up time. Institutions get relatively little patient care time from them, but support them for the long-term benefits of having new faculty up to speed at the start of their attending career when they will need to see more patients.
Now, why do they want this career? I suspect you would get better answers from them directly, although I also suspect you know what you'd hear.
Note that the issues for hospitalist fellowships are different and best left for elsewhere. This is specific to academic general fellowships.