When a person registers a domain name, they become the owners of that domain name. It does not belong to the registrar they hired to procure it, and they are not "leasing" the name from the registrar. A lease would suggest that the registrar owned the domain name prior to registration, which is not the case. And if you extrapolate, it suggests that some entity owns all unclaimed domain names de-facto, which is also not the case.
When you purchase a domain name, that does not award the registrar the status of "Actual Owner" of that domain. They are providing a technical service, nothing more.
ICANN should probably get involved and punish Network Solutions for this action.
Network Solutions might be within their rights to choose to no longer 'host' the domain name themselves, but in that case they must be obligated to arrange for a transfer to a different registrar. What they are not allowed to do is simply steal the domain name from it's actual owner because they disagree with the content of the owner's website, thus preventing a transfer.
It is important that we do not allow corporations to steal control or property from the open internet by using these politically charged circumstances as their theater of operation.