The corporate veil protects shareholders from being personally liable for the debts and obligations incurred by the corporation. Speech does not create debts or obligations, so the veil is not an issue.
Corporate speech is subject to some regulation, just like personal speech. Corporations can be sued for slander and libel, advertisements must meet certain standards of truthfulness, etc.
On top of the fact that it's practically impossible to make a libel accusation stick when political speech is even barely provable - being incorporated doesn't prevent a libel proceeding from moving forward. It just means that it's more difficult to pin it to individuals within the corporation.
Even then, there are ways that the corporate veil can be pierced to still go after individuals.
Corporate speech is subject to some regulation, just like personal speech. Corporations can be sued for slander and libel, advertisements must meet certain standards of truthfulness, etc.