Basically, it seems Microsoft designed Yammer so that any employee of any organisation can start/join a Yammer network for their employer just by using their work e-mail address. If the employer then wants to actually administer the internal social network that they've unexpectedly wound up with - for example, to remove ex-employees - Microsoft charge them a substantial monthly per-user fee.

Sounds more like it wasn't made clear what was or wasn't an okay internal communication tool, which points to a lack of training and IT management.

That's a pretty scummy business model, IMO.