I imagined an open group being like a public pub. When the cop walks in, the topic of conversation changes and the officer doesn't know what was said before she arrived.

It's open to all, but there's no NSA-style collection so they can go back through time when they fimd a person of interest and find out all of the people they talked to.

That's why I would want to encrypt it. To enforce the "you had to be there" aspect of conversations that people are accustomed to with in-person conversations. 

 Well, it's really easy to build a government bot, which just joins all new chats that are created, to passively collect all the info. And if the group is closed/«open not for everyone», then it's not an open, it's a closed group.

So that's not really making the spying harder for a cop. But if you wish, you still can setup self-deleting messages (eg. deleting the sent in a day), you don't need an encryption for that