"Cheat legal" - grammatically correct?

If you're thinking about the slogan—especially in the context of the product name—then the advertising agency responsible for the slogan has done its job. Advertisers have long aimed for the sweet spot in their target audience's psyche where a form of usage slightly rankles but doesn't prompt immediate dismissive ridicule. Previous winners in this game include "Deathsticks taste good like a cigarette should," "Drive friendly" (a Texas Department of Highways slogan), and "Think different."

Clearly, as Edwin Ashworth notes in a comment above, some adjectives-as-adverbs are so well established in everyday English that most people wouldn't bat an eye when exposed to them: "think positive," "work smarter," "breathe deep."

At the same time, others sound so odd that they would inspire immediate widespread rejection without getting the chance to burrow into their target audience's cerebrums: "communicate intelligent," "exercise religious," "gratify immediate."

Neither of these categories of phrases serves the advertisers' purpose, which goes to show that the issue isn't one of grammatical legitimacy or illegitimacy, but one of insinuation and surreptitious resonance. In short, the goal is to create the 2-second literary equivalent of an earworm—to formulate just-a-bit-off phrases like "dream unlimited" and "respond Pavlovian" that serve as the advertising equivalent of the "little critchers" crawling toward Lieutenant Chekhov's inner ear.