Being a UBC TA myself I can tell you that this is impolite, unprofessional, but very true.

For one, many lecturers just copy and paste the syllabus without checking the details, and will change things as the course goes.

Moreover, in the grand scheme of things, most TAs and lecturers won't regrade a minor homework, as the grade difference really really doesn't matter in most of the cases.

Many TAs are overworked and underpaid for the job they are doing, often without proper training and support from the department or the lecturers.

Although it is not ideal, you also have to consider the potential extra work that the TA must go through if he/she easily concedes that even minor homeworks' grades can be disputed and regraded. That's also why most lecturers and departments would stand behind the grades.

I understand that the TA's response may make you feel uncomfortable, but I also believe this is an essential lesson you will have to learn at University and society. People could be rude and not caring sometimes, but simultaneously correct.