The TC in ThinkPads need proprietary software do be really dangerous, but that's not the TPM Stallman fears. That's Palladium (or whatever the last name) where even Free Software wouldn't make any difference, because the control starts at boot in hardware before any software is loaded.
I object to both of them, because both attack our freedom in ways that we cannot overcome by developing and using free software. The TPM is designed for "remote attestation", which enables a web site to check whether you are running the "official" DRM-afflicted software. If you are not -- for instance, if you have installed GNU/Linux instead -- then the site simply refuses to talk to you. And the "official" DRM-afflicted software won't let you redistribute whatever you got from that site.
The result is that there is no way to talk to the site from a machine running free software. It is not just hard, it is not just illegal in some countries whose governments are against their own citizens. It is impossible.
That is what makes treacherous computing so dangerous.
You are right that the essence of treacherous computing is encryption that uses keys not fully under the user's control. However, just installing your own key does not overcome the problem. Doing that won't enable you to talk to the web sites that do remote attestation. On the contrary, it will make sure you can't talk to them. They know what key your machine is supposed to have, and if you have replaced that key, your machine will never work with those sites.