I and a friend of mine had a strikingly similar experience.
This happened in the first year as a graduate TA.
My friend, let's call him J, and I, along with other first year TAs were assigned as graders for Calculus II.
We were given quizzes and exams together with some rough grading guidelines, and we were responsible for coming up with partial credit schemes for problems for which students had show work. The professor generally trusted our work and didn't really bother us... until the end of the semester. After reporting the final grades, the professor called for an emergency meeting. We had to come back to campus around dinner time to re-examine the final exams. The professor asked us to pick out the exams of all those students who barely made a C. She then reviewed these exam in detail and questioned our grading decision problem by problem. We were being way too generous, in her opinion.
Eventually, she got tired, and unlike us, she actually need to eat dinner.
So she left and require us to finish regrading before we can go home. Even though she did not say explicitly, the message was clear, we had to turn as many C's into D's. Note that for this course, D was basically a failing grade at least at the time because most courses that require Calculus II as prerequisites would require "C or higher" anyway. So basically, we were asked to fail as many students as possible. We got the message, and redesigned some partial credit scheme. Eventually, we turned a huge number of C's into D's (I don't remember the exact number, but it was certainly well over 100).
After that, we felt terrible about it. Not only our professional opinions were not taken seriously, we also felt bad for the students, and most importantly, we felt we became a part the academic mafia.
So far our story is almost identical to OP's, right?
I wasn't a person with strong moral foundation, so I felt bad for awhile and then just ignored that. My friend, J, however, just couldn't let it go. A week or so later, J confronted the professor on her shady behavior and eventually reported this unethical procedure to the department head, which is what most of the answers (so far) are suggesting the OP should do (at least morally speaking).
In my story, however, my friend J didn't do very well. My reporting, J didn't become a hero. Indeed, J turned out to be very wrong.
It turns out, the department head already knew about and approved the professor's re-examination/re-grading of the final exam. The professor felt our grading was too generous for a long time, but she didn't/couldn't correct in time. It was only after the final exam she realized that a few new TAs' generosity would allow hundreds of students pass Calculus II without actually being sufficiently competent. So the final exam was the only chance for her to correct that.
It also turns out, she did what she did only because the department head had been nagging her about this overly high passing rate.
It also turns out that the department head started to worry about passing rate being too high only because the physics and engineering department had been complaining --- the math department produced a huge number of students who cannot handle maths used in advanced physics and engineering. Indeed, the physics department threatened to open their own "advanced mathematics" course to replace Calculus II.
It also turns out that physics and engineering department complained about their students' poor math foundation only because the dean of the college decided to address the high major-change rate for physics and engineering --- many students study physics for a year or two and then get discouraged and switch to some "easier" major mainly because they stuck on mathematics.
I can keep going along this chain of events. At every step of the way, someone made a right decision: Surely the dean has to address unusually high turnover rate. Surely the head of physics department has to demand that their students have good mathematical training. Surely the head of math department has to worry about grade inflation. And surely the professor for Calculus II has to consider learning objectives and not just counting points.
Of course, as new graduate TAs, we couldn't see any of these higher level objectives... we were only worrying about partial credit assignment one problem at a time.
So I guess my point is...... well, actually, I don't have a point. But my and my friend's experience could serve as a reference. The OP should consider the possibility that he/she (especially if he/she is a new TA) may not have access to all the information. There is a small possibility that the professor is trying to correct a disconnect between grading scheme and learning objective.
I'm sharing this story because I thought about this experience a lot lately as I start to see the difficulty in translating abstract learning goals into concrete grading schemes, and the OP's situation is almost identical to what we experienced. In my story, my friend J wrecked his career: soon after that incident, he lost his TA position. With no one taking him as RA, he was forced to drop out.