I think you are correct. It is professional courtesy. McCain took a huge gamble by selecting her as his running mate. Put her on the national political map. She is doing the right thing by supporting him.
Yes, McCain put her in the spotlight. Then later, when the campaign was over, he remained silent and let Sarah twist in the wind for over six months while various of his own "campaign advisors" stood in line with the media liberals to take turns beating on the poor woman like some arctic pinata. When Palin resigned her governorship and was finally able to respond to her aggressors, that big wuss McCain finally found his voice.
I hope her support of McCain is equally tepid.