Why is it illegal for a the police to use dragnet surveillance but it's ok for a private company to provide dragnet surveillance and then for the police to just

Why is it illegal for a the police to use dragnet surveillance but it's ok for a private company to provide dragnet surveillance and then for the police to just buy it? Isn't the just obscuring the origin of the data for the same effect? If you obscure your finances through the same kind of redirection we often consider that fraud. Why is this not fraud?