I don't think UA resells Amtrak Business Class, so when you try to book business class ZFV-LAX it's pricing you a United business class EWR-LAX segment and then adding on a flexible economy ZFV-EWR segment to complete the fare. That's part of why it's so expensive.

And while there are many exceptions, one rough rule of thumb for ZFV fare advantages is to look for routes where UA's EWR-XXX nonstop service is a monopoly or fortress route (especially if they're the only ones with a nonstop from all of NYC), but where somebody else flies PHL-XXX nonstop that UA might want to undercut with connecting ZFV-EWR-XXX service. LAX is less likely to have great deals on ZFV fares compared to EWR fares because multiple airlines offer NYC-LAX nonstop service in competition with UA, so the EWR fares aren't so inflated.