I waffle between wanting to point out to people that many "First Amendment" issues don't implicate the constitution because there is no government action. But I also recognize that the First Amendment is an ideal that extends beyond the legal restrictions it imposes on government. It is short-hand for the idea that different view points, religions, ideologies, etc. should be given a place in the "marketplace of ideas."

So, yeah, as long as people understand the distinction I think it is fine to invoke the First Amendment even when there may not be a government action. Just don't claim that First Amendment guarantees your right to do something in those situations, or prevents others from opposing you. . .