When should the grading scheme for quizzes and exams be finalized?
This seems perfectly reasonable to me. In this type of freshman physics course, there are some very common difficulties that students have:


Physics is counterintuitive, e.g., students often intuitively believe in some kind of half-baked aristotelianism, and this belief can be resistant to instruction.
Students try to solve problems by shopping for a formula that has the right letters in it, and then plugging numbers into the formula. They don't think about what the formula means, when it's true and when it's false, etc.


This is all very well documented in the pedagogical literature. The best summary I know of, although it's out of date, is in Mazur's book Peer Instruction. Mazur shows a lot of evidence that physics instructors are in denial about what their students are really learning and about the (in)effectiveness of traditional teaching methods. There is a huge gap between how physicists think about their subject and how their students think about it.

A lot of these issues are qualitatively different than in other fields, such as mathematics. E.g., I don't think students have deeply held intuitive preconceptions about derivatives and integrals before they walk into a freshman calc course. In a course like freshman calc, a lot of what the students are learning to do really is just algorithmic -- there's a reason it's called calculus.

For these reasons, it can be extremely difficult for an instructor in a freshman physics course to write appropriate exam questions. Results can be really horrible if you write questions that require conceptual understanding, multistep problem-solving, symbolic rather than numerical calculation, or interpretation of symbolic results. You can write problems that just require plug-and-chug, but then you're dumbing down your educational standards.

Because of the gap between how instructors think about physics and how most students think about physics, there is a real danger when setting an exam that you will be taken by surprise by how horrible the results are. This is particularly likely to happen with inexperienced instructors. Fiddling with standards after taking a look at the answers is IMO not an unreasonable way to prevent this kind of meltdown. Especially for a non-tenured instructor who can't afford bad teaching evaluations, I think it's pretty understandable to do this. As time goes on, people usually get a better feel for what kinds of questions will be hard. If I were evaluating someone for tenure, I would rather see them doing the kinds of things described in the question than see them writing plug-and-chug exams.

There is one thing that does seem a little odd to me about the specific situation described in the question. If I'm understanding the question correctly, the instructor in charge of the course seems to have delegated the writing of some of the lower-stakes quizzes to TAs, but the instructor will still be the one writing the final. However, there doesn't seem to be any mechanism for making sure that the standards and style are consistent throughout the course. Ideally students would be doing homework that is similar in style and difficulty to the quizzes, prelims, and final. The instructor should be setting the tone as to what should be the mix of easy and hard, concepts and mathematics, numerical and symbolic. If that isn't happening, then something is wrong.

Daniel R. Collins wrote in a comment:


  In future semesters adjust the tests so that's not needed. Personally, I don't find it hard to dial in an appropriate difficulty level after the first semester.


If these questions are being written entirely by TAs, with almost no supervision by the instructor, then it seems unlikely that there is any institutional memory that would allow this type of adjustment to occur over time. If that is the case, then the serious problem is lack of supervision, guidance, and institutional memory -- not a particular practice in grading quizzes.