/r/dankmemes has similar subject matter but it's clear that it's ironic and it's black humor.
They are two fundamentally different things. Nobody is being indoctrinated into being a fascist from the latter. The former is actively attempting indoctrination.
Maybe you could make a utilitarian argument against black humor. Maybe you could argue that such jokes encourage actual baby killers and the world would be better without them. I really doubt that's true, but even if it was, it's a very different argument. People want these kids expelled because they think they are actual monsters, which is probably false. Not because of theoretical arguments of the utility of banning black humor.
Pols transition to conservatism has nothing to do with black humor, but how it's set up. It's a site that not only doesn't censor, but strongly encourages discussions on taboo topics. Of course it will attract people that believe taboo ideologies and are banned or ostracized elsewhere.
I, and I expect the person to whom you reply, don't adhere to that assumption. I think that "just a joke", in this context, is the exhibition of profound naivete. And even if the speaker realizes it's a joke--the listeners often don't. See the /pol/ commentary elsewhere in this thread; /pol/ was "just joking" until those jokes were taken as articles of faith and became the core of a literally fascist movement.