Perhaps in theory (emphasis on the perhaps, IANAL and neither is the author), but not in practice. Nvidia has a great deal of power over ATI, Asus etc. They have no power over Dell, much less Apple. Nvidia needs Dell and Apple far, far more than Dell or Apple needs Nvidia (they don't).

Nvidia and AMD are selling, if not identical products, then near enough for government work. If Dell decided to drop Nvidia from all their product lines, the impact on Dell would be negligible at worst, but it would be a catastrophe for Nvidia (while AMD would be making out like Scrooge McDuck). Apple? Nvidia sends Apple the corporate equivalent of drunken post-breakup apology voicemails (releasing Mac drivers for cards that aren't offered on Macs).

Nvidia's ability to enforce branding guidelines on Dell is nil. If Dell decided they'd rather brand "Nvidia" cards as "Dell GraphicsMaster 6000 SUX," Nvidia would have to roll with it.