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
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
The problem with killing people who don't join you when there's other options is that eventually that gets out.
I can easily see the current White Council manipulating events to try and convince reluctant prospects to join. But they'd have to be very subtle about it. Because.....someone who IS capable finds out, and joins another group out of spite. Now what? They're protected, and they start spilling the beans about how the White Council manipulated them, killed their dog, threatened their family...you get the idea.
"Join us or Die." And then that nice guy with a spare coin goes, "Wow, that's pretty rude. Now, here's our offer. You don't have to take it; just hear me out, and if you want to walk away afterwards, no problem. WE don't kill people who say no."
"Join Us or Die." And then this Faerie Princess ambles by, contract in hand, which would guarantee your safety. (At least from the White Council...)
"Join Us or Die." And then...but you see my point. This could work for a while; but eventually, someone is going to get mad enough, stubborn enough, cantankerous enough, and last just long enough to join another group. And then they're going to talk...a lot. And maybe start offering their own rewards for joining them...
For that matter, this COULD be the origin of The Black Council...maybe they weren't always the bad guys.
They also have the knowledge you need to develop your powers to their fullest extent - who else would have that? Okay, let me rephrase that - who else would have that and be willing to teach it for such a low price? All the White Council charges is membership. They don't insist that you use your magical powers one way or another, they don't have you churning out enchanted items, they don't have you breaking the laws of magic in soul destroying ways - all they insist upon is that you become one of them.
Of course for some that's too high a price. And they can't be everywhere so some people with great talents slip by them - I'm thinking that Kimmler was never a member of the White Council.
As for the infighting, politics are happening. The sort of politics that happen in every office and twice as much in the halls of academia. They aren't really fighting - they aren't killing each out, training assassins and giving them enchanted weapons, or doing anything like that. They'll hurt each others feelings and careers, but no one ends up dead. For more on this, all I can say is read Turn Coat. Do that and you'll discover the council's worse nightmare is that the infighting goes real. Once that happens the council will be broken and mankind at the mercy of the monsters.
Richard