They say it shouldn't be legal. The first answer on the FAQ:

"Whats happening?

Cards Against Humanity is exploiting a legal loophole to pay America’s blue-leaning non-voters to (1) apologize for not voting last time, (2) walk us through a step-by-step plan of how they’d vote this time, and (3) post “Donald Trump is a human toilet” on social media. This whole thing should probably be illegal—so quick, give us your money before they change the law!"

The entire FAQ is worth a read, it's pretty funny