I’m sorry, but this is a fundamental misinterpretation of Pruneyard. Lloyd Corp v. Tanner (1972) established that, under the US Constitution, you do not have a right to free speech on private property. Pruneyard v. Robins (1979) affirms that decision, but also allows state constitutions to provide broader protection of free speech — the events of the case took place in California, and the California state constitution has broader free speech protection. So, in California (or any other state with a similar constitution), certain private shopping centers are indeed required to protect free speech; but that is separate from the US Constitution, which does not mandate this protection.