Okay, so they buy extra bandwidth. How much extra? As far as I know they can get more bandwidth only at the start of each month. So they add say 20%. The demand is still higher, but they do not know how much higher. So they have to wait another month and buy another say 20% extra. Eventually they will reach the point where they have enough and the problem goes away. And no I do not think they can jump in large chunks, because I think there is a penalty if they do not use within a certain percentage of their purchased bandwidth (can someone confirm?).
Now also look at it like this, they have a small customer base and not many of them are torrenters. It does not take many of them leaving (or getting Freeview like exportgoldman said) to drastically change the demand for bandwidth. However, if they sign up more torrenters, then the fluctuations in bandwidth requirement is reduced.
So perhaps they do want to become an ISP for torrenters as they will use a lot of data for which they get money to pay for bandwidth, it is just that they do not yet have enough torrenters to stabilise the bandwidth requirement. Or perhaps they are working on a deal for bandwidth with their current or a different provider. Who knows, but they will definitely not disclose such information and also can not predict when negociations are completed.