They absolutely did. AOL in particular used a buffer overflow bug in AIM to execute arbitrary code to determine if the client was actually AIM or a third-party client pretending to be AIM.
MSN Messenger and Yahoo IM both cried foul about this and called out for interoperability, but then turned around and denied third-party clients access to their own networks once they got big enough.
MSN Messenger and Yahoo IM both cried foul about this and called out for interoperability, but then turned around and denied third-party clients access to their own networks once they got big enough.