Whether it's true or not, it's quite possible that none of these other teams are offering him the money we think they are. If anything, it makes the most sense for the Celtics to give him a long contract. Unlike a team like Cleveland or New Orleans who is trying to build around young stars like James and Paul, the Celtics have a relatively narrow time frame. If the Hornets don't get Posey and fall short next year, it's no big deal. For the Celtics, falling short is a big deal. There might not be a next year.
Furthermore, if Posey is around a year or two more than he needs to be, it'll probably be in the early stages of rebuilding. If a team like Cleveland signs him, it could hurt their chances of winning down the road.