There is no such thing as "unlimited data", your speed is your cap.
Anyway, it's understandable that your ISP will want to have a talk with you if you push/pull 10+ TB every month (in this case, you're probably violating their "no commercial service on consumer line" rule), but 250GB/month is just ridiculous.
I can see that you're a bit out of date on these issues, so let me inform you:
1) No, your speed is not your cap. Net providers are increasingly implementing "soft caps" that they don't tell you about when you buy the service.
2) If you think 250GB/month is ridiculously low as a cap, go try to use 10TB a month with most providers and let me know what your results are.
I don't mean to sound like I know the best pricing scheme for ISPs but it has always seemed to me that the pricing model used for home internet makes the most sense. Once you have an infrastructure in place, the cost to send data over it is minimal, right? So, to me, paying for speed instead of an amount of data makes the most sense.
You are assuming they have the infrastructure to support 100% usage from all customers at all times. They obviously do not. So they try to discourage / limit usage to avoid having to pay for more / newer infrastructure.
They should decide whether they want to bill for X amount of data transfered or X amount of bandwidth and clearly advertise the service as one or other.
BT have been able to do this in the UK with their Infinity FTTC service. No small print about "Subject to FUP" - it's really unlimited, and fast (I get 80Mbps down and 19Mbps up).
Checked my bandwidth usage for the last month (we stream the majority of our viewing) and it stands at 955GB. Not so much as a peep from them about it being excessive.
So they should provide totally unlimited data for a fixed price?