They are certain to run the machines at 100% continually, which will cost more than a typical customer who doesn't do this, and leave the old machines with less second-hand value for their auction thing afterwards.

I’d bet that main reason would be power. Running machines at 100% doesn’t subtract much extra , but a server running hard for 24 hours would use more power than a bursty workload.

(While we’re all speculating)