@11

Title 18, § 333 of the U.S. Code actually requires, "...intent to render such bank bill, draft, note, or other evidence of debt unfit to be reissued..."

While the degree to which these bills have been marked may make them technically unfit for reissue, I doubt you could make make a case the the intent was to make it unfit, as presumably the artist wants the bill to remain in circulation.

As a Where's Georger, I frequently encounter the misconception that marking U.S. currency is, in itself, illegal.

See also: chocolate dipped ika.
Not bad, really, if you're the type to indulge in tentacled comestibles.