How do you compare two "A"s, one from a mediocre place and one from a well established institution? By knowing the difference, of course.
Where I grew up, and A was a 93. Where my partner grew up an A was a 90. We already have this problem. For better or worse your GPA + your universities prestige/perception/etc are a factor in hiring/grad school anyway.
Is a 4.0 from University of Phoenix (Online School) the same as a 4.0 from an Ivy?
If the outcome was the teacher talked about that lesson, then said "we'll adjust the grade so you don't all have negative consequences b/c you did nothing wrong: this is a school, after all!" yeah I'd agree with you...
In fact, that is the difference between a good educator and a bad one: a bad one will just say, "it is what it is", and describe their failure to teach as laziness or stupidity on the part of the student.
If the knowledge is necessary, it sounds like there should be an additional class before said class to give knowledge that can be built on in the more difficult advanced class.