They do not block unupdated phones because that will force users to buy exceptionally expensive devices to keep updated. Another thing is actually supportability. Just like any enterprise software, it targets a set of common devices so they can provide support and QA on it.
Now, that isn't to say there aren't security issues, nor that QA is perfect on devices they do support (like with any enterprise software projects), but it is a valid reason from a business perspective to not support everything.
Why do you think generally OpenBSD and such platforms aren't supported by commercial software, nor most open-source software? It adds a lot of burden for testing and QA on developers who will need to become intricately familiar with yet another platform.
People stare blindly at some customer support answers and hinge everything on those words, but if you think carefully about how software, compatibility and support work, this all makes perfect sense.