For the former, viewing or downloading it encourages the sick individuals making it to create more of it, since it brings them hits for their sites or (in some cases) literal money from paid purchases or subscriptions. In that case, consuming it any way encourages the more harmful production. It provides an incentive to create this stuff.
That's not true of standard copyrighted content. In those cases, distributing it (legally or otherwise) is not encouraging something illegal to be done since producing said content isn't illegal to begin with.
TLDR:
Distribution or downloading illegal content is bad, because it encourages people to do something illegal to provide said content.
Distributing or downloading legal content isn't bad because creating said content isn't bad to begin with.