No. WWOOFing is defined in the convention against illegal work as "active and fair holidays". It is an unpaid activity in agricultural areas. It is not work but an exchange, an act of solidarity, and a discovery. There are no obligations of productivity, no subordination nor hierarchy, and no pay.
The WWOOFer is there to discover, learn and voluntarily and freely participate to the daily activities of a host who wants to share his knowledge. Note that the MSA and WWOOF France have finally agreed on a common definition of WWOOF (in French).