Just like Skype. From their T&Cs
"4.1 Permission to utilise your computer. In order to receive the benefits provided by the Skype Software, you hereby grant permission for the Skype Software to utilise the processor and bandwidth of your computer for the limited purpose of facilitating the communication between Skype Software users. "
Comments elsewhere (just google for ... Skype bandwidth usage ...) indicate that in some circumstances the usage can become serious, particularly on some capped bandwidth connections.