Ok, I haven't read all the posts in this thread, but here's my take.
A kid comes in and makes a 90% on a test. The kid is a good kid and tried and studied hard for the test. The kid has a low A (90%) average so far.
Next week, the same kid comes in, and for some reason has a bad day, and makes a 49% on the next test.
Since a 49 equals a 0 in this lady's class, the kid now has a 45% average in the class (averaging the 90% between two tests).
If the kid gets to keep the 49% the kid would have a 69.5% average in the class. There's a whole lot of difference in a 45% average and a 69% average. The 45 would almost be impossible to recover from before the end of the nine weeks, especially for a kid that is struggling, but working hard in the class.
So, in my opinion, the teacher, giving 0 points for anything under 50% is stealing points from the student.
If the kid is just a lazy, good for nothing, student, then he'll continue to make failing grades and will fail the class. Let the kid hang himself.
If the kid is a good kid and had a bad day, this could really blow the wind out of their sails and set a bad tone for a good student.
To take away points that the kid earned, is to me, wrong.
JMO