There could be a lot of reasons, I commonly find these to be one of the reasons:
1) One man show (no staff to pay)
2) On an unstable/overselling provider (UnitedColo, etc) who does 500gb-1tb of transfer for $99-199 a month
4) Overselling (which usually in the ends bites them in the ass (I know from personal experience))
If it's not one of the above, I'd be curious how they can sustain a real profit. I don't consider $20-100 a machine a profit, but I guess some do.
What does one host say to the other? "(HostA) Want to go see a movie?" "(HostB) Sure, can your parents drive?"
I'm premium, and no, I did not have to pay $6 a month to figure that out.