You should be aware that there are technical aspects with regard to QA that people in engineering and purchasing--especially purchasing--might not be aware of. QA review of POs, especially for new products, is almost always a good thing. I can't tell you how many times I've encountered issues and delays because purchasing or engineering overlooked something important. Also, "Not my job" is almost never a good response to a request for help, regardless of the situation.
I was not being asked to help.
Getting engineering to work with purchasing removed the need for checking the POs because of preventive actions in engineering.
Yes, the whole cross-functional team needs to be working together to deliver quality assurance; QA cannot do it alone.
We also used to say "there is no such thing as a quality problem" to stop the instinctive call for more checking or more inspecting by QA.
John