I think you are interpreting "legal personhood" in a manner that is incorrect.
"Legal personhood" means that the corporate entity is a legal person in the matter of contracts and lawsuits. The "person" of the corporation is just a facade (in the design patterns sense) which provides useful simplifications in contracts and lawsuits.
You could completely replace corporate personhood with contract law. You would just have 10,000 page contracts that would specify the liability of each individual shareholders is limited to X, a lawsuit must target all shareholders and damages/debts are limited to Y, etc. Basically, each contract would reproduce corporate law.
Then whenever you wanted to collect a debt or sue the company, you'd need to dig into the contract to figure out how.
Instead, we've created a useful simplification/standardization: you say "Company X owes me $50,000", and it's up to Company X to figure out internally how to deal with this. It's also standardized to a few specific forms (LLCs, LLPs, S-Corps, C-corps, etc).
That's all corporate personhood is. A corporation has no inherent rights except those rights granted it by it's shareholders. Similarly, your computer has no rights, but that doesn't mean the government can pass a law saying "your computer may not criticize Obama." Such a law doesn't violate your computers rights, it violates your rights.
Similarly, laws against political speech violate the rights of the owners of the corporation who are ordinary flesh and blood people (as opposed to legal facades to simplify lawsuits and contracts).