WWOOF is a means to Green tourism, friendship & experience.
About Japanese Immigration
Via WWOOF travelers are able to contact local people, correspond with them towards beginning friendships with them, and then go to visit them (just like Pen Friends used to do prior to the internet), in regions all over Japan.
Approx 20% of WWOOF participants are overseas tourists traveling on a normal tourist visa (otherwise called a 'Temporary Visitors Visa'), who participate in WWOOF as part of their holiday itineraries. Additionally, approx 25% more are non-Japanese people who are already reside in Japan, such as exchange students, or people employed by Japanese companies, or people with permanent residency. WWOOF as a means to friendship, travel & experience has entirely nothing to do with money or employment. Full details about the many aspects of WWOOF can be seen elsewhere on this website.
Below is text copied from the the page 'A Guide to Japanese Visas' at http://www.mofa.go.jp/j_info/visit/visa/index.html
under the heading 'Activities authorized to engage in', under 'Visa Category', and 'Temporary Visitors Visa':
Sightseeing; recreation; sports; visiting relatives, friends, or acquaintances; visiting a sick person; attending a wedding or funeral ceremony; participating in athletic tournaments, contests, etc. as an amateur; business purposes (such as market research, business liaison, business consultations, signing a contract, or providing after-sale service for imported machinery); inspecting or visiting plants, trade fairs, etc.; attending lectures, explanatory meetings, etc.; academic surveys or research presentations; religious pilgrimages or visits; friendship visits to sister cities, sister schools, etc.; or other similar activities during a short period of stay in Japan.
Other visas such as Working Holiday Visas, etc., can too be used to participate in WWOOF in Japan.
Regardless, Japanese Immigration Officials at ports in Japan retain the right to let in and turn away travelers entirely at their discretion. The website for the Immigration Bureau of Japan website is at http://www.immi-moj.go.jp/english/index.html
The website for The Japanese Ministry of Foreign Affairs that stipulates visa types is http://www.mofa.go.jp