Well, diskspace is still somewhat expensive in the ranges DH is offering them. You can't just take the price of one 500gb hdd and assume that the cost of offering you 500gb at DH. If you actually use 500g, you can be almost certain that at the very least two copies exist of those 500g, since at DHs quantities, disk churn becomes a real problem -- you expect harddisks to fail, and you don't want your customers to even notice. If you assume $100 for one 500gb hardisk, you are already talking two of those to handle 500gb of actual data (plus backup overhead; in the worst case this will be another 500gb-1000gb, but in the average case it will a WHOLE lot less and almost negligible. For the sake of this argument, assume two 500gb discs for 500gb of storage (=$200). That's the cost of the harddisks. You are also going to need a case to put them into, rackspace to put that case into, and controllers to drive those drives in that rack. This can easily add another $10-20 per harddisk, possibly more. Then you are going to need to get power to those harddisks, cool the racks, and make sure you have UPS. This will be even more costs.
$250-300 initial cost (and this is optimistic) per 500gb block, plus quite a bunch per month to keep those running smoothly isn't too far-fetched. So a single customer using all the provided space will easily cost DH a bundle of money.
Thing is, the vast majority of users are not using 500gb of space, nor even close to it. Some are (and that's fine -- good reviews, recommendations, etc. come from those people), most aren't. Even if you are currently in the latter category and suddenly need 500 gigs of space -- no problem. There is always enough spare capacity to handle a few people doing this over night. There is never enough spare capacity to handle EVERYBODY doing this overnight, but that never happens.
Bandwidth follows similar rules. The initial costs are already sunk when you set up a server (and it's a bit harder to spell it out in numbers per customer); usually, however, a single server has a 100mbit interface to the DH network. If you WERE to use 5tb of traffic, you would use an average of around 16mbit/s of that (and since traffic is almost never average, you'll probably be using twice that during busy times of the day). It just takes 3-5 customers doing this to max out a single network interface. Then again, this almost never happens; the vast majority of sites on the internet use less than 1 gigabyte of traffic in a month, let alone 1 terabyte. Similar things to disk usage apply.
(it is also quite a lot harder to prove that your network interface is congested and you are not able to even use all your traffic in a month; this MAY happen, but probably doesn't very often. However, proof-wise, slow speeds can often equally well attributed to broken routes or upstream problems DH has no control over.
Also, DH could not handle it if, overnight, all their customers were to start taking advantage of all that traffic afforded. Then again, that never happens.
