17 USC section 102(a): "Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression . . ."
This section includes a list of eight "works of authorship" (literary works, musical works, dramatic works, pantomimes and choreographic works, pictorial, graphic, and sculptural works, motion picture and other audiovisual works, sound recordings, and architectural works), but that list is not intended to be all-inclusive. In addition, the categories are broadly construed, as for example to sweep computer programs and compilations into the definition of "literary works."
Thus, there is nothing preventing legal language from being copyrightable as long as it constitutes an "original work of authorship." And, if you go to any site that offers legal forms for download, you will find the vendor claiming copyright in the contracts being offered, thus presumably precluding others from taking the same contracts and re-selling them.
That said, the kinds of legal language and contracts you are talking about are almost never made the subject of copyright, I think, simply because they are usually not "original." Most clauses for indemnification, etc. are recycled over and over. Lawyers themselves will often lift them for re-use.
You are right to be concerned about the potential risks in this area but, in practice, I have never heard of any large website complaining of copyright violations from use of legal language similar to theirs and I think the practical risk of borrowing their language is low. For much of the standard stuff involved, I would think it would be nearly impossible for them to prove that the language used is "original" as required for it to be copyrightable.
Moreover, to sue for federal copyright infringement in the U.S., the copyrighted material needs to be formally registered with the copyright office. I would seriously doubt that any large website bothers with this formal requirement over a matter about which they normally could care less (other than making sure it protects their own legal position as used on their site).
I don't know of any definitive place to find a discussion on this but I believe this gives you a pretty good overview of how this area generally works. You can and should check with your attorney about details for your specific situation.