Character Voice in Narrative

I'm reading a novel right now, and I have this thing where, when I start a book, I HAVE to finish it. Even if it's totally terrible. This one is terrible, and it's proving to be really difficult. A lot of me wants to shake the author for the following reason:

If you write your first person narrative character as an American, their voice SHOULD BE AMERICAN. Doesn't matter if you, the author, are British, or if the book is published in the UK. If can't make your character American, DON'T MAKE HER AMERICAN. An American would NEVER say "The third 'series' of Lost"....Series in American English means the entire show. It's the third SEASON of lost. We don't wear trousers unless we're old or it's formal, they're pants. We don't wear jumpers ever, they're sweaters. Disneyland is in California, and Disneyworld is in Florida. If you say Disneyland and you mean the one in Florida, you're wrong. We don't often "shout" at people, more commonly, we "yell" at them. Etc, etc, etc, etc.

I realize the author is British, and as the book is published in the UK I understand the use of British punctuation and spelling standards, but character voice should be true to the character's nationality. The character in question is American, but does not sound remotely American. Worse off, she doesn't even sound neutral...she sounds distinctly British. I just have to wonder, why bother? The character being American isn't even particularly important to the story, so I have to wonder why the author bothered and didn't just write the character's nationality into the voice she's clearly more comfortable using (British). Nothing wrong with that...I don't think I could write a convincing British character. The point is to know your limits and stick with them.