I generally think of magic as the extra/different set of laws of physics that the fantasy setting's universe has, and that if rules aren't stated to exist, that just means the protagonists don't know what they are, or perhaps the people studying magic don't know either. Given that fantasy settings usually have medieval tech, that's not all that unbelievable.
If a setting has unrestrained magic, I call foul. Magic with no rules or principles over how it works means anyone who can do magic can cast any spell they want, or even any spell they imagine existing, but there's no rule saying it obeys what you think or say or convey with your magic wand gestures, so the spell could do or be anything and nothing. Meanwhile, everybody and nobody is magically gifted, and by the way, where's the rule that says only sentient beings can do magic?
BOOM! The pebble you just stepped on just cast meteor swarm!
A setting with unrestrained magic shouldn't look like a medieval fantasy setting, it should (on a good day!) look like a world that was sucked into the warp and is now ruled by the Chaos God Tzeentch. On a bad day, it doesn't resemble any kind of world.
Yeah, my definitions of "principled" and "unrestrained" might be a bit different. I prefer principled magic because unrestrained magic is a horrible idea, and leads to horrible places and no stories.