For one, I think it is irresponsible to leave math graduates so woefully unprepared to enter the workforce. There are not enough PhD slots for everyone to attend graduate school, nor should they. Math students can be very effective in the workforce with some practical programming experience under their belt.

There are pedagogical reasons too. Programming has been an endless soure of applications and motivation to learn and explore math, especially computer graphics and FP. Many math students are naturally curious and explore on their own anyway, but of course a good instructor can accelerate the learning process.