Especially in the sciences, there is a shortage of American citizens going into graduate school. It's all over the country and all over the disciplines. It's hard to get graduate students from this country. The changes in the training and testing of international students who hope to teach are part of university-wide efforts to improve the quality of undergraduate education. Better training also enhances the experience of international graduate students at the University.
International students are eager to improve their skills.
Many departments, including math, insist that their foreign graduate students meet the University's standards for teaching within one year to remain enrolled. Graduate students also need the money they earn from teaching to support themselves. And they know the ability to speak fluent English will be an advantage when looking for a job. They have all their graduate work, they have research, they have exams, and then they have this, the language and teaching skills training. Supporting the students while they qualify to teach has created an extra challenge for some departments.