Because the rate is not released by the hotel. Some agents undercut the rates, or floates a rate that is supposed to be marked up, naked (without mark-up) to the web. Asking the guest to book from the OTA is to see what voucher the hotel receives, and catch that agent as the culprit who undercut the rate without hotel's consent. The agreement with hotel and agent is that hotel can refuse to honour the voucher. In this case, the agent has floated a rate not discussed with the hotel. Hotel asked the guest to go ahead and book if there's a lower rate in OTA. Guest books, hotel has the right to refuse to honour the voucher.