Twenty years ago, ICQ and AIM were two different, incompatible, networks, and AOL did everything it could to prevent third-party clients:

From http://www.linux-mag.com/id/902/ :

But AOL has always been adamant about not wanting their users’ screen names and their passwords going through any software that’s not controlled by them. It’s an understandable position. I don’t know if it gives them the right to try as hard as they do to block everybody.

From https://nplusonemag.com/issue-19/essays/chat-wars/ :

AOL was causing the client to look up a particular address in memory and send it back to the server. This was tricky, vastly trickier than anything they’d done so far. It was also a bit outside the realm of fair play: exploiting a security hole in their own client that our client didn’t have!

And they even used trademarks on terms like "instant messenger" and "buddy list" to fight competition: https://www.wired.com/1999/05/does-aol-own-buddy-list/