An answer to this question: Yes, this is normal for virtually all major hotel chains.

Hotels use stay controls in order to maximize profits and ensure there are rooms left for multi-night guests. If the hotel believes there is sufficient demand, they will only offer to sell rooms that meet those stay controls. For example, when you made your reservation, your rate may have been subject to a minimum stay requirement of 3 nights.

If the night you're dropping is a "slow" night, they may not be able to re-sell it. Thus, they don't want to offer your initial rate because it means they will lose money. You can view this as a contract. When you make a 3 night reservation, you expect the hotel to provide a room for those three nights. Conversely, the hotel will expect you to stay in the room for those three nights at the agreed upon rate.

If you need to change your reservation, you're basically forced to meet the then current restrictions. It's usually spelled out in the rate details under the "Modifying Your Reservation" second.

For example, here's a screenshot of the rate details for the Fairfield Inn & Suites Spokane Downtown: