If you look at the definition of a Liberal Arts College, then both USNA and USMA meet that definition -
which emphasize undergraduate education and award at least half of their degrees in the liberal arts fields of study.
The question many folks have is what constitutes "Liberal Arts". Liberal Arts is not just English. Liberal Arts (also known as "Liberal Arts and Sciences) includes English but also Chemistry, Bioloby, Physics, Mathematics, Economics, English, History.
Traditionally students who went to Liberal Arts colleges took a wide variety of courses in all these subjects.
The term Liberal Arts is does not equate to a politically left wing curriculum. Rather it goes back centuries to define the education worthy of a free person.
Majors such as Engineering, Management and Business are actually professional majors, as opposed to Liberal Arts.
While both USNA and USMA have an engineering component required of their curriculum the majority of the student body does not major in these subjects. Rather they embrace a well rounded "Liberal Arts" curriculum.
USAFA, on the other hand, is not considered a Liberal Arts College. I believe this is because a majority of their students major in Engineering or Management - which are "Professional" programs. In this sense, "Professional" means a program that trains one for a specific job or profession.
Incidentally, until 4-5 years ago USNews put USNA and USMA into the category of Engineering schools. They only competed there but I guess they looked and the majors they offer now and how many students are in a liberal arts major and moved them appropriately.