Some developing countries are very sensitive about foreigners entering and taking jobs from locals. A short visit of 1-2 months is not a problem, but longer periods are. WWOOFing is viewed as Agro and Eco tourism and for this reason visitors can WWOOF to experience the country, culture, cooking, organic farming and lifestyle in the country. Some visitors try to hide paid employment as volunteering as a result some countries require a "work permit" to volunteer for "extended periods" as over 2 months. Work permits take 2-3 months to process, require lots of paper work, a letter from the prospective employer verifying the job and they cost about $300 usd.
As a WWOOFer you are always visiting as a guest and can enter most countries on a tourist visa as long as you are staying for a short period of time (less than 2 months).
This is a general reply and some countries may have specific rules that differ, check with your local country WWOOF coordinator for specific details on their country and/or the (FAQ) Frequently Asked Questions section of their website.
Hope this helps,