Unfortunately your silly rule is something that exists (not for interior decorators of course) but for countless other trade jobs (barber, plumber, etc). Whether that's good or bad I can't say
I personally see it as good. Why wouldn't I want someone I trust with my hair or pipes to not have something to vouch for them?
It's only a downside if you see cost as the most important thing about all else. The clear consequence is that a trained barber/plumber will require higher compensation to make up for the training, and due to less supply since not everyone will be able to get a license.
It's unambiguously good, and that's coming from the perspective of someone who is routinely frustrated by regulations around residential plumbing and electrical work. It would be utterly insane to remove minimum credential and testing requirements from trades where fucking up results in catastrophic damage to a structure, fires, etc.
