No, not all non-accredited schools will necessarily be diploma mills. There are a wide range of schools out there that don’t have national or regional accreditation but which provide high-quality education and training and are state licensed to provide educational services. Accreditation is voluntary in most states, so schools who do not seek it out aren’t necessarily offering a substandard quality of education. The majority of schools that will not be regionally accredited are those which provide specialized or vocational training in a particular field, but some other larger educational organizations do exist without accreditation.
That said, it is important to be wary of non-accredited schools because there is no way of knowing with certainty whether a school is a quality institution or not. Additionally, without accreditation, a degree from this kind of institution may be worthless in the job market wasting both the time and money of the student.
And yes, some non-accredited schools will be diploma mills. These unscrupulous organizations dupe students out of their money, promising degrees for little or no work. Accredited or not, any school promising something too good to be true is one you should stay away from.
If you have questions about any non-accredited school, do your research before committing any time or money. Most sites maintain lists like this one, found on the Department of Higher Ed for the State of Maine. On it, students can find a list of schools that are non-accredited and any reasons why they were denied accreditation so you can determine which may not be worth the time and effort to attend.