This was designed for exactly that sort of scenario. If you're runnin a SAAS then this is totally common practice, it's used by at least AWS, Slack, and GitHub off the top of my head.. Are you saying these companies are doing something stupid?
How could you phish github or aws by using their dns/cert setup as they intended it to be used?
The fact that you can create that domain name to resolve is funny. I can think of a few phishing schemes I could use with this, if I were so inclined.