Hmm...I think the differentiating line is in: casuals vs competitive.
In casual play sitting back and throwing projectiles continuously is looked at poorly because its casual play, you're expected to take losses in casual play to make things fun.
In competitive play it becomes zoning because now your projectile spamming has purpose and meaning behind it.
Distancing and spacing are another thing. If you are mindlessly spamming fireballs across the stage is one thing. When you are firing off fireballs in your opponents face is another thing. If you are close enough that using an alternative attack (that would be considered more effective) would be preferred and yet you are not (read: you are being an idiot), then you are spamming.