The Westing Game is almost weirdly patriotic. (There's evidence Raskin wrote it after being inspired by the 1976 bicentennial: see this link.) Can you think of another murder mystery that's so proudly American? Many of the characters are recent immigrants or have been altered by the immigrant experience. Westing himself is an immigrant, and that may be one reason why he's so adamantly in favor of America and the great things this country can do. He has first-hand experience with the kinds of opportunities and liberties that the country was established to offer. Let's not forget, the will's clues are chopped-up lyrics from "America the Beautiful" – as the characters puzzle over and worry about their clues, they keep getting pushed to language that praises the country they live in.
The Westing Game's apparent focus on wealth and identity allows its author to slip in a steady stream of propaganda about how wonderful America is, which is especially significant considering it was published so soon after the conclusion of the Vietnam War.
While The Westing Game doesn't shy away from the hard issues about American life – it comes down hard on prejudiced viewpoints of race and class – it does a wonderful job of illustrating the opportunities the country provides for immigrants and entrepreneurs to start over and build the kinds of lives they want to live.