求教, 为什么读个part-time MS签证会有问题?

录了UW-Seattle STAT MS之后想double一个CS MS。但是UW CSE没有一般的CS MS program， 只有一个不提供F1的professional part-time (evening) master program(简称PMP) 主要是开给附近IT公司在职员工进修的，上8门课拿到学位。

其他的申请条件满足，但是PMP的小米说:

While your case is extremely rare, I believe if you are in the country on a student visa you would not be eligible for the PMP as a regular or concurrent student. However you may be eligible to enroll in our individual courses via the steps described previously.

不太理解为何持有F1的full time graduate student去上一个part-time master 签证会有问题？
part-time master不是和小学生上奥数补习班一种性质的么？

求教{:4_112:}