They're unable to stop all mixers, so their existence is a given. What if they'll use this to create a few honeypot mixers?
Who says they haven't done that a long time ago? Realistically, such a service could be an ideal tool for what
"no such agency" has been doing for over 10 years when it comes to BTC. The goal is always the same, to convince someone that they are improving their privacy, while on the other hand they are actually losing it.