I don't construct the right as free speech for corporations. However, corporations are groups of people. They have individual rights which they can presumably exercise as a collective as well. To say otherwise is to say that: A pamphlet by John Smith is protected, but a pamphlet by Citizens for Sovereignty (repr. John Smith) is unprotected.

The issue seems to come at size. Corporations may be large groups of people who are not ideologically connected but economically. Then one influential person within the group may claim to speak on behalf of all of them without confirming that even most of them agree. This is more difficult for me.