Actually it's not considered OK these days by the majority of responsible breeders. Originally, it was thought that a bitch couldn't care for more than six puppies, so large litters were culled down; during the Wars, economics came into play - rationing was only provided for potential war dogs, and whites were not a part of this group. Over the next few decades, it became a common thought that white Boxers were prone to several major health problems, and euthanizing at birth was considered more kind than allowing them to live with a severely impaired and possibly pain-filled quality of life. Today, we know that the only health issue that is more prevalent in white Boxers is deafness, and that even that only affects a little less than one in five dogs.
(Now, in a technical sense whites are still 'culled', since they are removed from the gene pool, but these days it's done via speuter/non-breeding contracts, rather than euthanization.)