Your right its not a backflow preventer but a backwater valve. Similar but different. A backflow preventer is typically on the water line. If as you say the backwater valve is sticking open, you may have to find out why your getting backup. Whether you have a backwater valve or not, your plumbing should not back up unless there is an obstruction up the line preventing proper flow. A backwater valve should only be used to prevent sewage from backing up into the basement when there is a clog in the line from the house to the street, or when the street itself backs up. Though the valve was stuck open, the piping should always be designed to flow away from the bathroom at 1/4" per foot to the street. When it backs up, there is something wrong. If its backing up regularly, then you have to find the problem, not the sympton. This can be done with a fiber optic camera. It may mean, you have roots in the line, or a problem with your line. Or someone could have installed the plumbing in a convoluted manner.