"...considered 不礼貌. Why is this?"
Because 尊重 is normally used when you show your "respect" to or for someone even if you disagree completely with his or her point-of-view or actions. Much like "I disapprove of what you say, but I will defend to the death your right to say it" That is "respect" in this sense.
So, why is it considered 不礼貌 when using it as in -- 我尊重中国的工程公司?
My humble opinion is that it is read as "conditional respect"
And, perhaps in certain systems of thought, (or dare I say it, due to political upbringing), it is difficult to imagine that one could "respect" a 中国的工程公司, a Chinese engineering company, and not agree with what they do or what they stand for? I mean what is there to disagree or object?
However, I think that 不礼貌 is a bit strong here or also out of context perhaps?
尊敬, however is a different kettle of fish.
It is respect per se, i.e. in the context of respecting some intrinsic, inherent and definitely positive, admirable qualities in a person.
Like in most cultures, (perhaps it is politically correct to say all cultures?), old age or old people, teachers, doctors, etc, in general are "respected" in this sense.