The article is a write-up of an 8 year-old paper based on research from 10 years ago and extensively quoting papers from 20 years ago, so current definitions of expensive don't really apply
One way to think of it is that torrenting movies requires an (expensive) internet connection, where all you need for a DVD player is electricity and some DVDs someone else ripped.
I think the weirdness is calling them "expensive"; nowadays you can buy a new one for $25 or so.