You are absolutely correct. Covering the company's ass is half of IT's job. Providing services is only the other half. It's a tricky balancing act.
Legal problem #1 with IM is logging. In public companies, particularly in regulated industries, anything that has ever been recorded by any employee can and will be used against you, and the discovery will come at your expense. Everything that is said, but NOT logged, will be used as evidence of your malfeasance.
For this reason, E-mail systems are typically centralized, and users are prevented from pulling mail off the server, so that all e-mail can be retained for exactly six months, no more, no less.
With third-party IM services, that can't be done. This is the reason for the rise of highly restrictive internal IM - Lotus Sametime, Microsoft office communicator. With Communicator you can ban both logging AND copy-paste centrally, at the server. It's a terrible tool, but it controls the legal risk quite nicely.
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Google doesn't worry about this because they have more money than sense. (Also, not a regulated industry.)