The return of Paul Heyman to WWE has been a real delicate matter and really shows how much stroke CM Punk has at this point, as it was only a few months ago that Vince McMahon wasn't willing to entertain any ideas involving Heyman, not even Heyman appearing in WWE '12 as Brock Lesnar's manager. Throw in the fact that Heyman and Brock aren't close anymore and this whole deal was fairly tricky.
Heyman recognized that with Brock Lesnar's limited amount of dates, there would certainly be a useful role for someone representing Lesnar to keep Lesnar relevant while he is away. Heyman urged Punk to pitch the idea and Punk was willing as he's always felt Heyman saved his job with WWE and they have remained relatively close.
Punk made the pitch directly to Vince McMahon and Vince brought it to the creative team for discussion. Typically, most people would be afraid to favor any idea involving Heyman since Vince has always shot them down, but since it was Punk presenting the idea, people felt safe being in favor of it. I'm told even while Vince had sworn up and down to never bring Heyman back, he recognized that as quickly as they'd been burning through dates on Brock, it wasn't worth continuing to hold the grudge and Vince realized bringing in Heyman was a logical and inexpensive way to keep Brock fresh in people's minds.
Other than Vince, his inner circle and Punk, almost nobody knew Heyman was coming in. Brock himself didn't even know until a few days before and according to a source close to him, didn't care one way or the other. The feeling is Brock doesn't trust Heyman anymore and they aren't close, but he didn't try to stop him from coming in and was fine with it.