"Private companies are regulating speech (in their applications, through their services and websites, etc.) as they are free to do under the US Constitution. "
Technically, there is no barrier that restricts a corporation from granting free space, so it isn't "free" to do so, moreso that it was not addressed because at the time of the framing of the constitution, the bigger dissenters of free speech were government, and religion, backed by government (or being the government.)
I disagree that this was an oversight of the of the US Constitution. Newspapers and books were both things when the US Constitution was written and, in all the years since, we haven't seen any amendments that would force a newspaper to print articles or letters that might cost them customers. Publishers are not forced to publish books that they feel might tarnish or otherwise harm their brands.
Indeed, it's my position that such laws would in fact be infringing on the free speech of those private companies. In addition they might cost those companies money, making these hypothetical laws also anti-free market.
While not an amendment, there certainly have been provisions to compel entities from providing a forum for sides they don't want to promote. [0] See also the now-repealed fairness doctrine. [1]
Technically, there is no barrier that restricts a corporation from granting free space, so it isn't "free" to do so, moreso that it was not addressed because at the time of the framing of the constitution, the bigger dissenters of free speech were government, and religion, backed by government (or being the government.)