The BBC writes in British English, and that's how the word is spelled here (or, if you consider them to be distinct words, "mom" is not a word). Just as I'd expect the BBC to write e.g. "colour" even if quoting an American source.
My understanding of newspaper tradition was that quotes can always be edited for clarity or grammar. Some famous quotes go further, e.g. "Crisis? What Crisis?" was printed in quotes despite not being the words Callaghan said.
