We sell a lot of Dell and for base models, it is very economical compared to self built.
The moment however we add a few high capacity hard drives or memory, all bets are off the table and it's usually 1.75-4x the price of a white box part.
I get not supporting the part itself, but, had them not support a raid card error (corrupt memory) after they saw we had a third party drive.... we only buy a handful of servers a month - I can imagine this possibly being a huge problem for Backblaze though...