It is dependent on the contract. That said, agencies can pay you per your contract (and legally must) regardless of whether they can bill the hospital for your unworked hours. CVOR travelers are a scarce and valuable commodity, and contract exclusions can be made on a case by case basis. So yes, agencies that fight for you (and their own profit margin) are a good thing. Agencies that do not do this, do not recognize your value and you shouldn't prop them up.
Barring unusual circumstances (like a new program), case loads are entirely predictable - I can promise you the hospital analyzes trends like nobody's business. The cardiac surgeon and perfusionists get paid based on annual business, and we are a relative bargain compared to them.
Flexibility helps too. If you are unwilling to float to general, you are less likely to have unworked hours paid.
Finally, life perspective. Treat time off as a surprise vacation. Go do something fun locally, or farther away. As long as at least your stipends carry on, and your debt load is not high, you can turn this into lemonade! You may prefer mandated overtime, but just flip your perspective and go with the flow for 13 weeks.
Sure, I know all about call requirements. But that is really unfair to turn your $70 an hour total pay into $4 call pay and restricted to a 30 minute radius from the hospital. It is difficult to get recruiters/agencies to understand this issue, but just say no to slave labor. One thing to realize about your recruiters, the agency managers, and the hospital administration, is that none of them may have ever punched a clock and do not relate well to the working class (which is what we are). They get billable hours, but don't get that our incomes and life need to be predictable and stable like theirs.