I recently saw that microsoft exists, and has 79 questions tagged as of this writing. Upon checking, apple, amazon and facebook exist as well.
Facebook I can explain, since the company and the main product are identical. But what about the others? Why do we need tags for companies instead of their product?
I'd argue that these should be burninated:
Does it describe the contents of the questions to which it is applied? Is it unambiguous?
I would say No to both of these. It's unclear whether these apply to the company, or any product they offer. Most questions e.g. for microsoft are about a specific product they offer (e.g. Windows, Office, etc.), not the company themselves.
Is the concept described even on-topic for the site?
I would say No again. Microsoft is not about security. Neither is Amazon, Apple, etc. Questions regarding security on devices running macos or ios are perfectly fine and those tags are used well.
Does the tag add any meaningful information to the post?
No. A question regarding Windows 7 should be tagged windows-7 or windows. The fact that Windows 7 is a product made by Microsoft is completely irrelevant.
Does it mean the same thing in all common contexts?
Yes. Microsoft is a company. That much is a given.