Well, it's mostly a "camp"-mentality, partially formed of players who want to give themselves the benefit of the doubt by not playing PP maps at all so they can say "I could be way higher but I just don't play pp maps"
Then there's also the actual pp farmers who just want to rank up to give themselves a feeling of superiority by having such a high rank in a very low time, living under the illusion they're just that talented to "improve" much faster than everyone else, usually will also condemn any non-farm map as "cancer" or "shit map", because all they did was acquire the skillset needed to play said pp-maps.
If you're coming from League, you would call the PP-farmers "One-trick-ponies", simply because they can only really play those kind of maps, however this is the type of playstyle that will get you the furthest in terms of ranks in a short time.
And as in every game, there's also the unofficial ranking / pp-police that will call you out on this kind of stuff.
Obviously there are a lot of people in between. Some just don't care, some just play everything there is.
Funny enough, both groups mentioned above usually do this so they can think they're better than they actually are, whether it be by inflating your pp by playing maps optimized for pp-gain, or by said benefit of the doubt of never playing said maps.
edit: nice ava momiji