I think your question was rhetorical, but there's an answer:

Because it means that modifying the sign is not useful. (Unlike, say, modifying a speed limit sign.) Also, the fiducial can encode its orientation or position (perhaps in relation to other nearby signs... and could be a hash or checksum of its position, to save space), so the vehicle would be able to know there's a mismatch and thus mark the sign as suspect/unreliable if it was in any other spot or orientation.

There are other solutions to these. But the same problem you describe occurs if someone moved a human-readable sign (but without any way to checksum).

I think fiducials are not a panacea. They are just one additional data source in what needs to be a robust sensor-fusion approach. But they make a whole bunch of stuff in machine vision a LOT easier to solve. Machine-learning approaches have the same problems but with less opportunity to address them, less robustness, and more overhead.

I supposed location and information could be added to a sign along with a certification or checksum. As long as its done in a way that doesn't require exact placement, signs found outside their location are ignored.

...something like, "Forest St. Northbound CLOSED (lat,lon) <encrypted checksum>"?

The sign could easily be tied to location, no?