Click used the software to demonstrate how easy it is to gain control of the tools used to hold website owners to ransom. It used software acquired through internet chatrooms. The software controlled 22,000 computers which it had infected. "Click ordered its PCs to send out spam to two specific test e-mail addresses set up by the programme," said a BBC description of the programme's activity. "Within hours, the inboxes started to fill up with thousands of junk messages." Source: Out Law

The law says you can't mess around with other people's computers without authorisation. The BBC and PrevX did not have the permission of the computer users to send those spam mesages. Sending spam from someone else's computer obviously gobbles up bandwidth and will use up system resources. Even if the BBC felt the impact would be minimal - it doesn't make it right. Furthermore, at the end of this next excerpt you'll see that the BBC "warned" the users that their computers were part of a botnet. They did this by changing the desktop wallpaper of affected computers owned by innocent third parties to display a message from BBC Click. Source: Sophos