This is very smart on De La's behalf.
Most of the early catalog will never be available on legit download sites like itunes because of the samples clearances...way too many writers, publishers, and record labels involved.
Plus, they are one of the most consistently touring rap acts. I read somewhere that a industry figure call them the "grateful dead of hip hop".
I'm sure most of their income is from shows and merch sold at shows. It's probably hard to expose new listeners to their music when half the catalog isn't for sale online and if it was they wouldn't probably see a dime of it.
Makes sense to just give it away.