Oh yes they can. If I'm a consumer, I pay for my connection that is sold to me and held in my name. I have no right to "resell" or "sublet" part of that. It is usually spelled out in the Terms of Service or Acceptable Use Policy.

At the very least, consumer connections explicitly disallow "Commercial Use". That means that you can't use them to run a business. You can't use them to turn a profit. You can't use them to generate revenue. That means no running servers, and that means no "reselling 2% of your bandwidth" for a few pennies because some Euro-Trash-Stranger wants to borrow it!