I agree that:

1. You need to check individual cancellation terms - different countries have different policies - e.g. a lot of hotels in the US and Europe have the arrival day 6pm cancellation policy, whereas Hong kong and China has the 48 hours cancellation deadline;

2. If OP doesn't like the rate, don't book it or cancel it;

3. I saw in other forums here at FT saying that a lot of hotels change their cancellation terms for special events like NFL or Christmas, so it's not exactly foreign to us.

However I do agree with some posters saying the rate is misleading - I would be happier if they use the term Best Available rate. Using the word flexible is a bit over the edge IMO. I think Marriott usually uses the term Special Events (see rate rules) for these kinds of rates. They could easily use something like New Years Celebration rate to illustrate that the rate is not a usual cancel up to 6pm on arrival date.

It's not that difficult for IHG to make things a bit clearer.