a) Because he is an owner.
But, since I'm not a lawyer, I don't understand how he can be an owner of a band that he quit.
Good question, and it sometimes doesn't occur to me that not everyone understands how these things work. Here's a quick, really basic overview:
Think of the band as a company, because that's what it is. (specifically, Ytse Jams, Inc.) Because it is a small company, people wear multiple hats within the company. Shareholders own the stock, which means they own the company. And the company owns the assets of the band, whether it be the name, the other intellectual property, etc. So as a shareholder, Mike owns stock in the company, which in turn owns the assets of the band. So assuming the company is owned in equal amounts by the bandmembers and no one else, Mike owns 20% of everything.
As an officer and director, Mike is an
employee of the company. So he is also wearing the employee hat. The law says that officers and directors have to run the company for the benefit of the owners. So, one person can't just take over and run the company in a way that privately benefits him and screws over the other owners (unless of course they consent and give him control).
By quitting, it's the same as if you quit your job. You are out. You no longer have a job. You no longer have any rights
as an employee. But as an
owner, he still owns his stock and, by extension, his share of the assets of the company. As such, he has the ability to take steps to make the company look out for his rights, including filing a lawsuit if he thinks the stuff he owns isn't being taken care of or he isn't getting paid what he deserves when he tries to sell it back to the company. Not saying he is right, but those are his legal rights.
Anyway, that's a
really basic overview. Hope it helps.