This is a concept at least in some EU countries, that there has to always be one person responsible in terms of press law for what is being published.
Not citing sources doesn’t imply plagiarism, as long as you don’t misrepresent someone else’s research as your own (such as in an academic paper). Giving an account of news that you heard elsewhere in your own words isn’t plagiarism. The hurdles for plagiarism are generally relatively high.
Most regulations around disclaimers in the USA are just civil and the corporate veil won't be pierced.