If the bookmakers did not have such a right that protects them from dishonest matches, then all legal bookmakers would go bankrupt for a long time. There is a lot of situations where fraud (in sports) is obvious, but it is unrealistic to prove collusion. These rules help bookmakers protect their business.
But what to do if the result of the match exceeded all expectations and the bookmaker suffered losses. However, the game was fair. In this case, the bookmaker can declare the match as a contract and refuse to pay the bets.
How can those who placed bets get their winnings?
In reputable bookmakers, this does not happen. Firstly, there is a limit on the maximum amount of winnings and the office controls that at any outcome of the match this amount should not be exceeded. Secondly, the mathematics of this business is such that for any outcome, the office is always in profit (on average).
Most likely it is and your words are true. What I wrote about can happen very rarely. However, I am confused by the fact that a bookmaker can declare the results of any match invalid and return bets to participants. And to do this, he does not need the match to be officially declared invalid due to players fraud.