So, back in the day, when Magic defaulted to 6, it used to be that your Magic rating increasing past the current maximum was gated by Initiation (which added +1 to the attribute, or later +1 to the maximum rating). I'm not sure which edition caused this limitation to disappear, but I assume it was originally there to prevent the abuse of the system that the above comments describe now being possible. It was never originally conceived of as an attribute in the same sense as the others, and didn't have the same ability to be manipulated through direct Karma spends. Initiation used to include the boost in the package.
You got me curious, so I dug into my big library o' Old Shadowrun Stuff [see attachment] to refresh my memory:
2e CRB: No initiation. "If a character can use Magic, his Magic rating starts at 6, but is equal to the Essence Rating, rounded down." Magic is basically a derived attribution, like Reaction used to be.
Also, you can lose Magic points when you suffer Deadly damage or when you are healed incorrectly -- roll 2d6, if you roll under your Magic attribute you lose 1 point, if you roll snake eyes you always lose 1 point. With, at that point, no chance of ever getting them back. Ooof, that's cruel by modern standards.
The Grimoire: No idea, my copy appears to be irritatingly missing. But I'm pretty sure this is where initiation was first introduced and your description above is correct. Every initiation rank just added 1 directly to the character's Magic attribute, and that was the only way to increase it.
Awakenings: New Magic in 2057: mentions initiation but only in the context of additional rules for adepts and physads. Also very briefly mentions the concept of mysads (I had totally forgotten that) except they're called "physical magicians."
3e CRB: "If you have assigned a priority to Magic, then your character's Magic Rating is equal to his or her Essence, rounded down." No initiation rules, and I don't have any 3e splatbooks.
So it must be either 4e, 4eA, or 5e that made this change to treating Magic as a full-fledged Attribute in its own right. And, to be fair, that brings one very clear benefit - you can now have street-level mages with Magic attributes below 6, representing their lack of training and experience.