This is not true. Most US teachers have the equivalent of an undergraduate degree in mathematics. Some of them (myself included) have an engineering degree and several upper division math classes (for majors). My course background:
Calculus and Analytic Geometry I, II, and III; Differential Equations, Discrete Mathematics, Applied Linear Algebra, Theoretical Linear Algebra, Abstract Algebra, Euclidean/Non-Euclidean Geometry, Number Theory, Mathematical Modeling, Calculus-Based Probability and Statistics, History of Mathematics, and Applied Analysis. The total number of semester hours in pure math was about 40 (some of the courses were on the quarter system and others were on the semester system, which makes the number approximate). These are just the pure mathematics classes --- obviously there was more math in the other science and engineering courses. And I was with math majors or engineering majors the whole time, many of whom went on to graduate study in mathematics. I also performed well, with an A in every class.
Your example of graduate work in education is likely true, because most PhD programs in content area education focus on research and pedagogy in that area. However, in many graduate programs (Ohio University is one example: http://www.ohio.edu/education/...) if you get a PhD in mathematics education you are expected to have master's level competency in mathematics. This is logical from the college perspective, because it allows people with PhDs in math ed to help out the local math department teaching undergraduate courses. Most people who get PhDs in a subject area (like math ed) were high school teachers who already have the equivalent of an undergraduate degree in the area. Many schools (Ohio State as an example) require an undergraduate degree in the subject area first, and the teacher's license is then obtained through a accelerated master's degree program).