If folks want to be in the Republican Party, even if they don't agree with everything the Party stands for, that's fine, but they shouldn't try to mold the party to THEIR whims. They also shouldn't publicly repudiate the Party, if they don't agree. The Republican Party should stand for something that is DISTINCT from the Democrats, otherwise, why would anyone be interested?
What Meghan McCain and others want is to have a Republican party that makes it easy for them to keep their liberal ideas, without them being challenged. They don't like the idea that their Party stands for ideas that make their liberal friends uncomfortable, because that makes THEM look like the rubes they think conservatives are.