They're fining (~$140) and detaining people who don't follow the rules in France.
https://translate.google.com/translate?sl=fr&tl=en&u=https%3...
It would logically follow that if the quarantine is legal, penalties for violating it are also legal.
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Most of those cases are quite old, but they're still the ruling precedent on the topic as far as my (limited) knowledge goes.
For one that went to the Supreme Court in 1902 and continues to be cited:
Compagnie Francaise de Navigation a Vapeur v. Louisiana Board of Health
https://en.wikipedia.org/wiki/Compagnie_Francaise_de_Navigat...
Which might be reasonable if there are flagrant violators and things continue to get worse.
https://www.nytimes.com/2020/03/18/nyregion/Coronavirus-broo...
"Concern over the spread of coronavirus in Hasidic communities had already been raised by a series of large weddings held this week. On Tuesday, the Fire Department broke up one wedding celebration in Williamsburg that had more than 200 guests."
I know in my neighborhood, everyone is outside but we are all maintaining 2 meter distance. Some neighbors even brought up their grills and stayed seperate but can talk (at a distance).