I have never personally liked the argument "if you don’t like it then get out" because it tends to go against freedom. I disagree with President Obama and am very vocal about it. I’d hate someone to tell me "if you don’t like the president then get out". That argument basically sums up to "keep your mouth shut, your freedom of speech is not valued".
Dana Perino of FOX News tells the atheist community that if they don’t like "Under God" then they have the right to leave. I guess the difference here is that they are trying to use the legal system to attack something so fundamentally American like our pledge of allegiance.