All OK Now,
Please allow me to clarify the statement that "A Principal has no right to prevent parents from meeting and discussing the school, or even fundraising for the school". I believe the statement is accurate and I also believe we are in violent agreement.
Some PTOs are set up as an extension of the school itself. In this case, typically, the PTO uses the school's bank account, or uses the school's (district's) tax ID number (EIN). Perhaps the Principal is the leader of the PTO group or holds a formal, and significant, role. In this case, the PTO is not an independent entity (legally or otherwise) and you are correct that the Princ. has the authority to disband the group at his/her whim.
I have no data to back up the following assertion, but it is my belief that the vast majority of PTOs are set up as independent of the school - to one extent or another. On the one hand you have PTOs that have gone the full 501c3 non-Profit route (like mine). These are clearly defined as independent organizations from the school they support and have the documentation to back it up (Charter or by-laws, own EIN, Non-Profit Approval letter from IRS). Other PTOs are less formally defined, but if they have their own bank account (not under the approval of the school) and EIN, I content they too are independent organizations from the schools they support.
An independent organization is, by definition, separate from the school. As an organization it is only beholden to its members (within the boundaries defined by it's Charter). So long as the members of a PTO organization want it to continue, it will be able to do so regardless of outside influences.
You are absolutely correct that the Principal is the head of the school and has some authority over who can and cannot hold meetings within the school grounds. If a PTO is so shortsighted as to get into the situation mentioned above, the Princ. is within his.her authority to ban the PTO from holding meetings on school grounds. This would make it difficult, but not impossible, for the PTO to operate. The PTO could meet at a parents house, or at a local restaurant, or the library, or rent a hall.
The Princ could make it very difficult for the PTO to function, but she/he does not have the authority to disband the independent organization. Of course, it the PTo were so disfunctional to force a Principal to ban it from the campus, it is entirely likely that it will implode on it's own without much more of a push.