Netflix Streaming

I have a computer, a tv, a wii and two tablets that have netflix installed, when ever i try to watch netflix simutaniouslly at least three of the devices i get an error message saying that my netflix account is used on too many devises. Please stop playing on other device to continue. What is the maximum number of devices that can be use at once and why is it limited since it is a paid for service ? and how can i play on more divices simultaniouslly.