Here is the official response from united:

Given the circumstances, your request for a
reimbursement of the rental car charges you incurred is certainly
understandable. Because your canceled flight was uncontrollable, we do
not refund any out-of-pocket expenses our passengers incur. However, I
will forward a request to our Refunds Department to refund the unused
portion of your ticket from Chicago to Traverse City. The Refunds
Department will inform you the value of your flight segment ORD - TVC

3 rental cars last-minute, 1 way, with a lot of miles ran around $1K. I'm not sure how much UA is going to value a one-way ORD-TVC flight. Hopefully they will credit around $100, in which case we come out around even or slightly ahead with 11 people traveling on the reservation. It was shady of the gate agent to claim this would be reimbursed if that's not United's policy. I was under the impression that they may provide assistance/reimbursement for their elites under such circumstances.