I've worked for 2 large technology companies. The first was a big one down in Southern California and HR there was as you describe.

The other was a big one in San Francisco, and their HR was insanely powerful... for some reason. It was quite a shock to me but to a lot of others used to Bay Area startups they made it seem like the norm.

So I guess my point is that not all HR is alike and there is probably some truth to this HR negotiating business.