I don't expect many to agree with me, and realize that the influence of private sector on public space can seem subtle. But it exists, often in the enforcement of regulations regarding the approved and disapproved uses of the space.
The first observation one notices when visiting a privately managed public space is the enforcement of various regulations which are geared toward attracting a particular type of user, and dissuading another. I don't think I want to get into a massive debate about who the "public" is, but privately managed public spaces benefit the abutters if they differentiate between one type of user and another.
As for streets separating RKG from abutters, that has little to do with the private management of space. Post Office Square serves its location incredibly well. But to suggest it is managed as a public space no different than Boston Common is incorrect. Park activities in Post Office Square are highly regulated and enforced to match the needs of abutters and their target users.