Why would the animals believe what Squealer said about Boxer's death?
The animals would believe Squealer's explanation because they have accepted his explanations about everything else. It's not as if the animals have rioted against Squealer's explanations about the fruit and milk mash, the retelling of the Battle of the Cowshed, the lies about Snowball, and about how the pigs actually "sacrifice" for the other animals. They have accepted his rewriting of the Animalism commandments and the commendations for Napoleon. The explanation for Boxer's death is another in a series of explanations in which the animals simply accept its plausibility. The animals would believe that Squealer says about Boxer's death because they have accepted the premises he has offered on nearly everything else, so there is little to believe that this would be any different. In the end, the animals would be able to accept this explanation because they have accepted much of what Squealer has said. In the end, this becomes Orwell's point. If the people accept freely what government says without question, it renders itself into a position whereby everything is accepted and little is debated. In this, there are problems with the relationship between the governed and those who govern.