I think what you are describing is called slavery!
Yes maids can get up to no good but so can the people that hire them. They are just people like you and I and should be treated with the same respect.
Yes there should be boundaries - the same as you would ask of anyone you employ. The same as what our bosses ask of us and that is that we respect them and do our jobs well.
Put the shoe on the other foot and ask yourself if you were not allowed to leave your company premises how much you would like that.
Just because you can do it doesnt mean you should do it!