Sametime? Run far far away. It is the most bloated client I've ever used for any chat protocol, it crashes frequently enough and when it does it will sometimes prevent the user from rejoining a group chat, requiring a new one be made and everyone move over. There isn't a way for people to join a group chat on their own accord and must be invited, nor is there a way to auto accept invites. Any time you need to copy/paste a chat log it must be manually edited so it becomes even remotely readable and some of the GUI settings work contradictory to what you'd expect (like disabling smileys, it just does not work).

Afiak it is just a subsystem of Windows Server thus requires no additional licenses. But there is some bizarre ass license subsystem (of Windows Server) that the summary refers to. I'd suggest reading the ToS.

IANA Windows SA.

Becase many nerds have a 'hacker' mentality, where if they purchased a physical device they own it and can do anything they want with it, DRM, DMCA, vendor-lock-in and other such evils are viewed as evil because they inhibit hacking.