They do have the right to change the award chart, but there is a tipping point at which it would be considered fradulent.

Many years ago, in the late 80's or early 90's I think, as the loyalty programs realized the monster they had created, they did major changes to awards. The state attorneys general got into the act and there was some sort of general negotiated settlement limiting how much they could change. I know for example that around 1992, 93, 94 or something like that, my United miles were split into "old" miles which could be redeemed at the old rates, and "new" miles at the new rates. I think old miles converted to new miles in 1997. The old charge was two first-class tickets USA to Austrailia for 120K miles. (Although back then, I think "F" still meant the same as domestic F is now.)

One thing that stimulated that was the 1988 debacle. Eastern Airlines, running on fumes, had a promotion that if you flew one Eastern flight in the first three months, you got triple miles on every flight all year. All the competitors matched it. That was also in the day before CC bonuses, so the inflation in miles in "circulation" was very significant.