OK, so I admit that the situation you're referring to is, to some degree, a problem. However, you lost me about here:
Of course, what they should get on that question is zero, if not an outright loss of marks.
As far as I can tell, you propose that should a student get a question wrong, they should not only not get points for that question, but they should lose points from questions they got right?
If this is what you're saying, then how on earth does it make
any sense? You penalize wrong answers by not giving points for that answer, not by taking away points the student has already earned for other, correct answers.
Students most certainly must earn their grades, but on the other hand teachers must give students the grades they deserve.