Archive use of third party services like AIM or something is technically challenging for a variety of reasons, particularly as the industry continues its slow but steady march towards SSL usage. Products exist which can penetrate this encryption and log the network traffic directly (when I say "challenging" I mean just that, not impossible), translating it into logged IM entries (and yes, they do it by obtaining the keys from the services in question), but they are awfully pricy compared to just declaring that IM shall not be used, and there are enough protocols out there with enough usage that there's some frequent pain around losing logging with protocol updates. You can also host an IM server internally in which case logging is trivial, but many people aren't even aware that this is an option, or don't want to deal with IT to set it up and maintain it, or don't want to purchase the off-the-shelf products which can do it but are
also more expensive (virtually by definition) than just declaring it shall not be done.
I used to work in this field. I do not recommend trying to break into it, there does not appear to be a market here.