I note with no objection that you dropped offlist, but I'll move back anyway.
Quoting Michael Paoli (michael.paoli@berkeley.edu):
I'm thinking the option may also be generic, but whether or not it actually does anything may be TLD specific, so, e.g. may do nothing for com., but may actually be usefully effective for some other TLDs. I.e. com. may not even use or accept "Billing Contact" into whois data, but some other TLDs may accept such.
FWIW, I also see that Billing Contact isn't shown in public WHOIS for unixmercenary.net, either -- but that's consistent with your hypothesis, since both com. and net. are operated by VeriSign, Inc. as "authoritative registry", or, as VeriSign calls it, the firm's "naming services division". https://en.wikipedia.org/wiki/Verisign#Naming_services
That line of business it acquired by buying Network Solutions in 2000, divesting the _regisrar_ portion in 2003 but keeping the registry (wholesale & back-end) portion. They also, ugh, have the contract for the root nameserver operation, operating two of them, "A" and "J", directly. Plus maintaining the root zonefile.
NetSol, in turn, got the football in 1993 under contract from NSF. The division between "registry" and "registrar" (within NetSol) followed in 1998 when ICANN got conjured into existence (at the invitation of Dept. of Commerce, IIRC), and ICANN grandly pronounced that it required NetSol to engineer an interface permitting competing registrars. (ICANN by itself was a "Who the hell are you?" paper tiger, but they had the blessing of US Dept. of Commerce Dept., which had legal and checkbook power, and ISTR somehow the issuance of marching orders somehow went from Commerce to ICANN to IATA (which is a nonprofit under Commerce Dept. contract, hence checkbook power).
VeriSign Naming Services is now at 12061 Bluemont Way, Reston Town Center development, Reston, VA.
Anyway, bottom line, I concur with your guess that "Billing Contact doesn't go into WHOIS" is probably implemented at the VeriSign com./net./etc. end of things, and is probably a legacy of NetSol 1990s buildout that's still embedded into everything, there.