Exactly.
Blanket rules are just as moronic as one rule fits all rules. They don't and they won't.
For example, the houses on my block have a high tension power line running along our property lines. Those are like 250' or so high (best guess, not getting a tape measure out to measure them). There is also a 300' water tower half a block to the west. We have trees around 100' ish. A NFZ starts at the south end of my block.
Flying my drone down our street to video Christmas lights (or halloween) at < 200' and violating (technically) a NFZ is not a problem for me. If there is a commercial aircraft flying down our street <300' then the pilot is yelling "Brace! Brace! Brace!" in the intercom! My copter will not change that or interfere with a crash in progress.
Would I be buzzing around at 400' or more in the NFZ? Certainly not. Again, reason needs to prevail, not one size fits all NFZ geo fencing.
As for the OP, its 400' above ground level. Otherwise most people would not be able to fly anywhere on the planet except near oceans where the ground is < 400' AMSL. Here I believe we are 600ish AMSL so that lets me get up to 1000' AMSL or 400' AGL. So I would think you could fly up to 400' above the side mountains but only 400 above the river. Technically. So it depends on where your copter is in that scene. Personally, I would buzz the bridge no problem. But I would also look out for stunt planes and manned choppers in the area as they COULD fly under the bridge. Not likely. But could.
Again, common sense. Fly where full size aircraft can't and you won't be in their way. If, at any time, you are flying where a manned aircraft could fly or would fly or is flying, then you are doing something wrong. Normally 400' is no problem. But if a crop duster is working the field, YOU need to move. Common sense.