Cena is the biggest draw and merchandise seller amongst women and children. The company apparently thinks that turning him heel will kill off all the money they make from those two demographics. Others think that while it may hurt those demographics, it would help in the male audience and would be viewed as one of the biggest heel turns ever.
We don’t really have any news yet on the reasoning behind WWE giving back the title to Cena. It could be because they thought their plan of taking it off him wasn’t working, that they didn’t like the results they were getting from Edge, or a number of other possibilities.
Cena has been working really hard since shooting to the top of the company, spending most of his days off doing P.R. stuff. It was previously reported that while the boos from the fans had been getting to him, he didn’t let it bother him too much and accepted that they can cheer for whoever they want.