In a state with in-person voting, if someone tells you they'll give you $50 to vote for their candidate, you can take the $50, go to the ballot box, and vote for the other person. They'll be none the wiser. In WA, on the other hand, they can watch you fill out your ballot and stick it in the mail before giving you the $50.
As a WA resident, our system is certainly nice. Is that risk worth it? Meh, it probably is.