Most US teachers have the equivalent of an undergraduate degree in mathematics.
I'll believe you, since I don't have any knowledge of US teacher training or US degrees. I do have a question that I hope you can answer.
How can that be possible? How can you have a degree in something else, and still have the same level of maths as someone with a maths degree?
For my maths degree, I had free choice for the equivalent of 1/2 of a year. Every other class was a maths class, although some over lap with the people studying physics and computer science (and the into stats class was general for a lot of degrees). There is no general education requirement for degrees in the UK, as that is assumed that you received it in high school.
Even for my free choices, I chose maths subjects, since there were more areas of maths to study that could fit into one degree. There were some areas, such as the calculus of variations, that I wanted to study, but I couldn't fit in.
So, if I, only studying maths for every subject in my degree couldn't cover everything, how can someone studying something else have the same level of maths coverage as me?