While documenting small changes probably is OK, I would think that reproducing long passages of a site's TOS, even in the context of documenting changes as a public service, could run into copyright-infringement problems.
Yes, some of these TOS have clear copyright notices. Are these valid?
[I work at Docracy] We believe that if everybody who visits and uses the company's site is implicitly held to these terms, then they should be able to have a copy of the terms. We would be very surprised if a company asked us to stop tracking these policies, and nobody has yet.
The copyright notices are valid, but fair-use exceptions still apply, and fair-use tends to be read fairly broadly with this kind of document, compared to, say, lengthy excerpts from a novel. I'm not aware if there's any solid caselaw specifically on quoting excerpts from contracts or licenses, though.
IANAL, but I think a big factor would be whether it acts as a substitute. Since they aren't being used as a TOS on docracy, then it shouldn't be an issue. If you copy a TOS and use it on your own site, then it could be a problem.