Precedent (regardless of nation of origin) is probably the second most powerful source of laws, right after Natural Law.
Fast forward to Valentine v. Chrestensen (1942), the supreme court unanimously upholds the common law that the first amendment does not apply to corporations. The court suddenly reverses in 76' with Virginia State Board of Pharmacy v. Virginia Citizens Consumer Council, Inc.
It was long held that corporations held some of the protections of the Constitution. Which ones was not so clearly defined. First amendment protection is now so defined, but that does not make it brand new legal doctrine. It's based on long standing legal precedent. It is a clarification of law.
You're making Valentine v. Chrestensen's decision much more broad than it actually is. Specifically, SCOTUS "ruled that commercial speech in public thoroughfares is not constitutionally protected" (https://en.wikipedia.org/wiki/Valentine_v._Chrestensen). Commercial speech has a very specific definition in law. It's about advertising.
The decisions later about Hobby Lobby have very little to do with Valentine v Chrestensen nor Virginia State Pharmacy Board v. Virginia Citizens Consumer Council, because Hobby Lobby's case was not about commercial speech.
Same with Buckley v. Valeo, and Citizens United v. Federal Election Commission because they were about political speech.