The allies didnt actually burn the books, they did something more sinister, they pulped them. Burned books leave ashes as evidence the burning occured and its kinda hard to hide them, but a pulped book can be repurposed into another document, effectively erasing its footprint. So they get to claim they didnt commit the largest book burning in human history (even if they basically did), because I guess the destruction of information only matters when you light it on fire and when the information in question is quack pseudo-science perversion normalizing sodomy.