I can't follow your logic dharvatis (maybe it's my fault).
Let's examine the sentence again.
Contractors should comply with (A) national and (B) Union legislation, (C) applicable collective agreements, or (D) relevant provisions in international instruments.
So the sentence structure is we should comply with (A) and (B), (C) or (D).
To simplify the logic, let's change "should comply" to "should purchase" and A, B, C, D above to an Apple, A Banana, a Cocunut and a Drum. And let's say "we" are the contractors here.
What you're propounding is (by deeming the "or" as being inclusive and equating it to an "and").
1) We should purchase an Apple, and a Banana, A Coconut or a Drum is the same as = 2) we should purchase an Apple and a Banana, A Coconut and a Drum.
I think 1) above could be interpreted 3 different ways - 1. we should purchase an Apple, a Banana and a Cocunut or 2.an Apple, a Banana and a Drum (as the choice is between purchasing a Coconut or a Drum) or 3. just a Drum (as the choice is between purchasing an Apple, a Banana, a Coconut or the Drum).
I feel there's a semantic (or syntactic?) ambiguity going on with 1) above that needs to be interpreted and clarified by the translator and the translator should not repeat the ambiguity, in my opinion. So I agree with Themi here, unless I could be persuaded otherwise.