Certainly "loyalty to McCain because he made her" is one possibility and the one most often used to describe her behavior.
But there is another possibility that has nothing to do with loyalty but rather that she is helping John McCain because he is the most powerful Republican in the Party. In other words she's kissing the GOP Don's ring in the hopes of future favors in return. This would explain a lot more of her behavior over the last few months-- donating to Graham and Hatch, supporting Steele, supporting McCain over Hayworth and cozying up to Mitt Romney -- than just "loyalty to McCain" would.
We all want to think of candidates as "belonging to us", but the fact is (especially on a national level) the candidates have to make their bones with the Party machine first, often doing all kinds of distasteful things that no one else wants to do. Only when they get to McCain's level do they get to do whatever they want. But until then, they follow orders, and Sarah following orders would mean that she is no exception.
I don't claim to have a crystal ball, but my guess is that the latter scenario is far more likely than the "it's just her loyalty to McCain because he made her" description.