I'm sure that's the same question the OP is asking themselves. Before talking about the manager, I think it's interesting the time that the meeting lasts. Because it's only 10/15 minutes, people would think that they don't mind arriving early so they can discuss before the shifts start, and things like that. Sounds normal, and it shouldn't be that big of a problem.
The problem arises when the meeting happens to be every day, and the fact that the employees cannot clock in after it's over… meaning those extra minutes they are there are unpaid.
And now, the manager… what is it with some bosses that they don't know how to boss? According to the way the story is described, this person is not only very disrespectful, but also has a bad way of encouraging participation in this meeting. On one side, he's saying it's voluntary, and that the people who don't feel like coming could avoid it. But that's not before clarifying that working is also a "voluntary thing" (sort of?), and that the ones who are not in the meeting aren't working that day.
I really don't know how someone can be so mean and so disrespectful. At the end of the day, they are employees working there, and there's really no reason not to treat the employees fairly and with respect. If this meeting is important to them, then he should find a way to make this a fun thing, especially if you're not letting them clock in beforehand, so they can feel encouraged to assist. At least, that's the way I'm thinking.
I do think that some people shouldn't be allowed to become bosses or managers, since they clearly lack the skills needed to be a team leader. Talking badly to people and forcing them to attend meetings is not okay, especially if you're the one also threatening to fire them for a single 10-minute morning meeting.