Then why did Harry's foster sister act dumb and fail the test?
I don't see them letting any powerhouses run around as rogues. Everyone important has to join or else... Well let's Tom didn't want to join. He later meets Dick, who doesn't want to join, and the two of them run into Harry (not Dresden, another guy named Harry) who doesn't want to join. The three of them start getting together as the Tom, Dick, and Harry social club - having dinner once a month. Then they get a few apprentices and before you know it the Tom, Dick, and Harry Circle is an alternative to the White Council. A small, insignificant alternative, but one of hundreds of little circles out there...
And let's say Tom, Dick, and Harry live in England and during WWII they use their powers to help the allies win while some German groups use their powers to counter that - before you know there's a wizard war and people are breaking the laws of magic for the best of reasons.
The White Council doesn't want that to happen. They can't allow it to happen.
One of their problems with Harry's mother is she wanted the council to get involved in social justice and that would eventually lead to splitting the council on national lines
. If the Wizards fight among themselves the Monsters win.
I see the White Council as akin to a medieval or early renascence Trade Guild. Do you know what happened to trades men who refused to join those guilds? Well, they joined, were chased out of town, or eventually killed. The glass makers were infamous for killing apprentices who might share their secrets. For a guild to work it has to have a monopoly on the trade or it will be undercut.
So yes, I see White Council as a "join us or die" type group.
But maybe they aren't open about it. Maybe if a young wizard-to-be refuses to join they manipulate things so that he's tempted to break the laws of magic and once he's done that they "kill the warlock" to make the world a safer place. Or maybe they are the only ones who can awaken True Power in potential wizards. Either way, if you're good enough then you're a member of the club whether you want to be or not.
Of course these are just my opinions, but they make sense to me.
Richard