I don't disagree with you at all, so let me first clarify my terms: By "challenge," I mean the interminable Internet "challenges" and challenges issued by teachers - exactly the "write a sonnet about ______" or even your "draw a pink pony" - screw that, I want to draw a rainbow pony, or a unicorn wearing roller skates! (I have, in fact, painted a painting of a unicorn wearing roller skates). The narrowly-drawn and "not invented in MY brain" challenges. But you refer to these as "constraints," and I agree with you.

I tend to think of structure (the sonnet) as imposing both a framework AND constraints, though. The sonnet, for example, has definitive constraints. (Actually, so do blank verse and free verse and I have no idea what most poems think they're doing, but I'd say about 80% of them are prose with cutesy line breaks.)

Constraints, then, provide their own problem-solving challenges to creativity. But constraints such as "draw a pink pony" are too narrow to pique much creative interest. Maybe we shouldn't call them structure OR constraint, but "directives." And that's really what most of those externally-driven challenges are. Directives.

CreativeCopyChallenge, though, just said "use these 10 words" and didn't suggest the form (could be a poem, scene, short story, non-fiction) or the subject. I often rebelled against the obvious "theme" suggested by the ten words. I think it worked best when they were completely unrelated - for me, the challenge, then, was to weave them into the coherent whole.