The game created a classic case of cognitive disequilibrium: Either the system is skewed and unfair, or the winners played unfairly. To resolve this by deciding that the system is unfair would call everything into question; young children are committed to rules and rule-making as a way to organize a community, and it is wildly unsettling to acknowledge that rules can have built-in inequities. So most of the children resolved their disequilibrium by clinging to the belief that the winners were ruthless — despite clear evidence of Liam and Kyla's compassionate generosity.
The game was patently unfair, so why did the children blame the winners rather than the system? I suspect there is a general tendency among people (not only children) to blame other people, as opposed to abstract causes. That's more important here than any positive commitment to rules and rule-making.