They have transfer limits and an associated overage fee iirc. I can still see this sort of thing happening if that is the case.
(edit) looks like that's not the case, I'm sure I used to have to buy a second instance a few years ago if I did want to use more bandwidth that was allocated
For those curious, the overage rate is $10/TB ($0.01/GB) after the transfer included in the plan.
The smallest amount of included transfer is 1TB for the $5/mo VPS.