It does sound indeed rather weird, but if you think about it, it makes a bit of sense.
So lets say they have 1500 of these $130 servers in their facility, total, that are unused. They offer 2TB per server, so that is 3000TB of BW.
When a customer pays the $130, it covers a slot that they have for that server. So let us say they sell those 1500 servers @ $130 each, allocated with 2TB bandwith. That means they now have covered colo bills etc for those slots and are done.
When you get extra bandwith, it is extra costs to them too. They probably have a set up plan with XXXXTB in their datacenter spread on XXXX servers..so if it goes correctly, they will profit a "Y" amount.
If you ask for extra bandwith, they owe their providers "Z" more, and it may mess up with any contracts/specials they get from the upstream providers.
Sorry for all the variables..just trying to point out something