While a corporation is a legal entity, a corporation cannot carry a gun. A corporation can not vote. A corporation can not do a number of things that a individuals can do.
A corporation is simply an association of individuals that happens to be recognized as a legal entity. I see no reason why an association of individuals should not have the same free rights as the individuals that make up the association.
I don't see how that argues against a corporation enjoying 1st amendment protections. The focus is on the incredible amount of things which a corporation CAN do just as individuals can. I see no reason why I should lose any basic right when I act collectively that I enjoy when I act individually.