I'm also noticing there is no explicit license on the official page. If it's public domain, attribution is not required. If it is not public domain, they should clarify the license (pretty sure this is indeed public domain).
For another data point, this catalog.data.gov site[1] lists the license as "us-pd" (ie public domain in the USA). But then yeah, like you said the attribution demand is invalid.
Ambiguity like this is way too common...