The idiom “more loyal than the king” was used by an author as a subtitle in a 2010 opinion piece criticizing a fellow Pakistani columnist for expressing misplaced concern on behalf of the United States over the possibility that leaks from within ISI (Pakistan’s powerful spy service) had revealed the name and identity of the then CIA station chief in Pakistan, in spite of the fact that the United States itself had not voiced such concerns through official channels.
In the first paragraph, the author of the piece states the idiom’s meaning as follows:
The idiom ‘More loyal than the king’ means someone who unnecessarily
behaves more concerned about matters that concern someone else and
ought not to have been of such concern to him. Generally it is spoken
about sycophants or those subservient to the concerned, for their
undue zeal and interest in matters owed directly to someone else.
(from ‘Pakistan’s Liberal Extremists: :More Loyal Than The King’ on ‘siasat[dot]pk)
Although the second sentence about how the idiom is generally targeted at unduly zealous sycophants would probably not apply in your context as presented, the idiom as defined in the first sentence would describe “Person C” in your scenario, with the United States representing your “Person B” and “sections of the US media” playing the role of your “Person A.”
Please note that variations of this idiom include “more royal/ist than the king” and “more papal/Catholic than the Pope,”
the latter being defined by Wikipedia (as an extension of its literal meanings) as:
Adhering more stringently to any norm (the norm of what is considered
offensive, in your case) more strictly than is required by the
arbiters of the norm (Person B, in your case);
with the former (as paraphrased from its use in ‘When the King Took Flight’ by Timothy TACKETT, via ‘Google Books’) meaning:
someone who “abides no compromise [when it comes to offensive language, in your case]” even when/if the king himself [Person B, in your case] does/would abide such compromises.