I would say that it isn't an "usual practice" on WWOOF.
There are hundreds of Hosts and thousand of volunteers and what is important is that everybody is comfortable.
If you are not feeling comfortable with all this responsibility, may I suggest you to kindly talk with your host?
Sometimes the hosts are so busy and the WWWOOFers are so nice that the host don't notice that there is a problem. I am sure he/she will understand your situation.