In the first case, they complain, and there's ~750 of them (in the course I TAed) so even a small number can take up a lot of time. The right way to think about it is for a small additional bit of time spent clarifying instructions you save yourself a larger amount of time later.

In the second case, it does depend upon how much the instructor feels it's their duty to uphold the integrity of the grades in their class. I'm not sure if I would have made the same choice in my advisor's shoes, but that is the decision he made.