What, after all, but a person, can speak?
The very concept of incorporation is to embody, to create out of some group of persons, whether a town or a guild originally, or later a group of investors pooling their money, a legal person which can have obligations and rights separate from the individuals who collectively constitute it. It is only as a 'person' a corporation has any existence at all.
But the idea that the legal embodiment of a group of persons, called into being to be a focus of rights of contract and to shield individual owners or members of it for its debts and liabilities, can have any opinion on any matter separate from the persons who own it, is nonesense. Worse then nonesense, it is an obvious impossibility, on simple physical grounds. Can anyone seriously imagine, say, the Caterpillar corporation disagreeing with its CEO, arguing with him on some point of social policy, and even going so far as to spend its money to rally public opinion against his view of the matter? To simply state the thing is answer it with 'Not just no but fuck no!'
Disallowing 'free speech rights' for corporations does not restrict the free speech of any citizen, or any actual person, in the slightest degree. All it does is require them to use their own resources to express their views, rather than the pooled resources of the corporation they own or direct, resources which are not theirs in the first place.
"The trouble with our modern corporations is they have neither bodies to be kicked nor souls to be damned."