Nexus interview Question

I just did my Canadian portion of the NEXUS interview at Pearson and they said I wasn't allowed to do my US portion at the airport, I would have to book a flight to the states. When I asked if I could just drive to the boarder, he said no and that since I did this portion of the interview at at airport now the other half also needs to be done that way too? This seems crazy.. Could I just rebook my interview for the boarder only? Anyone else have this issue??