I had a similar experience recently with the biography of another eighteenth-century political figure: John Keane's life of Tom Paine, in which the author insists on writing about Paine's childhood in 'Norfolk County', as if he grew up in Virginia, rather than East Anglia. Elsewhere, he describes Paine journeying into 'county Kent'.
How could authors who have obviously done extensive research into their subjects and clearly spent a great deal of time in England working on their books, make such basic mistakes? In both cases, you can see how it has happened: a familiar American usage has been transposed to an unfamiliar British setting, with jarring results (in McCarthy's case, he has clearly used as his template the way Americans talk about Congress, minus the definite article - 'in Congress', 'members of Congress'). But that doesn't explain why it happened: why these esteemed authors didn't realise they were in error, and why their editors didn't see fit to correct them. (This isn't meant as an anti-American tirade, by the way: I'm sure British authors make similar mistakes when writing about US topics.)
And why does it matter so much to me as a reader, to the point where I feel reluctant to go on with books that I was finding completely absorbing until that moment? It's probably because I think of reading as a relationship of trust. When I start a new book, I'm looking to see whether I can have confidence in the author: if it's a work of non-fiction, confidence that they know their stuff, and if a novel, that they're not going to surprise me with an unbelievable plot twist or unconvincing character. Once that sense of confidence has been confirmed (I find this usually happens in the first two or three pages), I can let go and enjoy the book, trusting myself to the author's care, rather as a traveller has to put their complete trust in a ship's captain or airline pilot before relaxing and enjoying the journey.
Coming across the kind of basic mistake that I noted above can be enough to shake that trust and make me lose all my confidence in the author - and in the book. If the writer can get wrong something that is so fundamental, then who's to say there aren't other errors lying in wait for me? Should I start distrusting the author's more important claims about their subject? Writing is a kind of magic trick, in which the reader is lulled into suspending their knowledge that these words have been created by a human mind, and for a time believes that they have somehow emerged from the ether and are an emanation of objective truth. Finding a mistake, however trivial, destroys that illusion and reminds us, annoyingly, of the author's fallibility.
Or is it just that I'm neurotic and obsessive?