Obviously there's some stuff they do that involves plants ("Lil Jimmy" with the cup of water at Capital Punishment last year) but most of time it's the wrestlers interacting with the regulars they know have traveled a long way to have fun and aren't a threat at all to their safety.
Cena has fun with those "We Hate Cena" guys because they're not a threat. They're just having fun at the shows and you realize that pretty quick when you see them there (and they're there often). Have you ever seen any of them do anything like you see with some psychos that have so much hate for a guy they don't even know? No. Never once have they tried to take a swing at him like that woman did to Jericho in the parking lot a few years ago.
The "Superfans" know the show is scripted and they enjoy it for what it is because it is fun to be there live. They enjoy the interaction they have with the wrestlers during the show and after when they're waiting at airports and the hotels for pictures & autographs. Check out "Brock Plant's" Facebook page and you'll see how many Meet & Greets the guy attends outside of WWE shows. He was there at the Ultimate Warrior signing I attended back in March. It's right around the corner for me, but at least an hour drive for him each way. We both were in line for at least 6 hours that night. He wasn't a plant that night, just a huge wrestling fan.
I know people here get excited here when they get a response on Twitter from someone like Punk (I do too). So imagine how it feels when you have a wrestler interacting with you during the shows and you have tons of wrestling fans telling you afterwards how cool that was and how lucky you were. Makes you want to do it again and again.
I know I've mentioned here before that at Night of Champions 2010 I was in the front row in Chicago with my sister and she had a "Cena, please throw me your shirt" sign (before they became common). She's super shy and had met Cena a few times before then. She's always been respectful to him. He's been great and I know he's seen us sitting ringside plenty of times before. It was the first and only time she's taken a sign to the arena and she only did it because I encouraged her to since we had front row seats. It worked and she got his shirt that night. It was seriously priceless to see her that happy at the show (huge Cena fan) and it honestly would've made a 2 day drive across the country to the show worth it. I imagine that's how a lot of the "Superfans" feel when they interact with their favorites at the show.
Here's another example...
"Frank the Clown"
A Superfan from the Chicago area. Kisses Punk's ass at the shows and on Twitter. He does everything he can to get Punk's attention, all the way down to tweeting just like him (music quotes, the "Lets Go!" stuff, etc.) and visiting the same places Punk does. He's the furthest thing from a threat you can have, just a hardcore fan that spends a lot of money and time traveling to the shows to enjoy them and interact with the wrestlers. He takes it a step further than most with the clown outfit to get attention, but he's just a fan like you and I. Punk knows this and occasionally interacts with him at the shows like the picture above. That was at the Summerslam Axxess event last year. Punk was doing a Q&A session there and decided he was going to bring the microphone to the fans and field questions. Punk knows the "clown" is from Chicago and flew out to be at Summerslam in LA. He even mentioned seeing the fan at the Money in the Bank show a few weeks earlier (the "clown" had the Straight Edge "X's" painted on his face that night). Why wouldn't you throw a fan like that a bone if they're that much of a fan that they'll put everything aside and fly across country to see you perform live on a big show?
EDIT: Notice "Brock Plant" is behind Punk there too as he and the clown travel to the shows together a lot with both being from Chicago.
