Entries for the Olympic Test Event were due by January 17, BEFORE US Nationals started. My guess is USFS entered McBeath/Parkman based on the likelihood that they would finish off the podium, at least behind Kam/O'Shea, Chan/Howe, and Efimova/Mitrofanov. Which was not an unreasonable assumption, based on past competitive performances...and also assumed by fans, given the predictions made by some on this very forum. When the final results deviated quite markedly from these assumptions, USFS was a bit stuck when it came to assigning 4CC given the competition timings. Could USFS have listed a substitute for McB/P for the Test Event? Maybe, but not sure they did. Could USFS have pulled McB/P entirely from the Test Event and not sent anybody? Sure, but at the risk of leaving a bad taste in the mouth of the ISU.
As for ISU ranking points being more valuable than a Senior B, that is true and especially for initial groupings at Worlds, but McB/P are so far down the World Standings right now, that any placement they would be likely to get at 4cc isn't going to move the needle much for their skate order at Worlds.
FWIW, the Olympic Test Event is to test the venue and operations using actual international competition conditions, to "shake out the bugs" so they can be fixed well before the actual Olympics. All venues will do these Test Events. The skaters actually entered into this competition may or may not end up in the Big Event.