Cena walked away because that's what the story called for. Storyline wise he probably WAS giving The Rock "his time". He applauded, just like classy Cena always does. In other words, it's exactly what the WWE wanted to have happen... And it did.
And storyline wise, The Rock called him back to make the character John Cena feel like a complete piece of crap.
But in reality, both guys were playing to the emotions of the crowd. They embraced their strengths and weaknesses witht he crowd and embraced them.
Storyline wise in my opinion, Cena walked away because essentially no one wanted him there, so he did the "classy" thing by walking away. And like you said storyline wise The Rock made him feel like crap by calling him back.
I just don't get the logic that Cena was giving him his time. He didn't needed it like I stated before. After the ppv ended for a good 7 minutes or so he even had more time with the fans. Any other time, Cena celebrates with the legends. Or he'll stand off to the side until he gets the ok from the legend. Why was it different now?
Think about it. You're not gonna stand around and wait to get booed, while someone else is getting praised like crazy.