Anyone breeding boxers, no matter how responsible, can end up with white puppies in their whelping box.
What people DO with these white puppies determines what type of breeder they are.
Responsible breeders who care about their breed, and steward it for the future, will require the spay/neuter of any white puppies, and any puppies who are more than 30% white, as this is not allowed according to the breed standard.
People who do not require the S/N of white boxer puppies, or puppies who have excessive white, do not care about the future or the health of the breed.
People who breed boxers who are more than 30% white are not stewarding their breed, and are ignoring the breed standard.
So what if they started out white. Every breed standard world wide frowns upon boxers that are more than 30% white, including the FCI standard, which is the standard of Germany, the country of origin.
Rottweilers started out a variety of colors too. That is simply an excuse for pups for bucks greed breeders to breed dogs who do not meet the breed standard who should not be bred.