A lawyer says yes. [UPDATE: Here's his letter.] The argument for infringement is actually moderately strong. Like most other documents, briefs are protected by copyright the moment they are written. The fact that they're filed in court doesn't waive any copyright. Lexis and Westlaw's distribution of the briefs is thus presumptively copyright infrigngement.
The question is whether the commercial posting of the briefs is fair use; and fair use law is, as usual, vague enough that there's no clear answer. I do think that the posting is quite valuable to researchers and to others who are trying to figure out what actually happened in a case, and why courts reached the results they did, and I think courts can consider this social value in the fair use analysis. It's also quite unlikely that allowing such posting would materially diminish the incentive to write good briefs, or the market value of a good brief; that too is potentially relevant to the fair use inquiry. But the case isn't open and shut, because there are no precedents (at least that I know of) that are clearly on point, because the various fair use factors seem to cut in both directions, and because fair use analysis is so vague in such situations.
Thanks to Kevin Gerson of the UCLA Law Library for the pointer.
UPDATE: A reader suggests, "I should think that the minute they are filed they become public information; accessible to all." That something becomes publicly available doesn't strip it of copyright protection — the point of copyright protection is largely to prevent copying even of material that is publicly available. So the fact that they are descriptively accessible to the public, at least if the public is willing to go to court or to read the briefs on PACER, doesn't resolve the copyright infringement question.
Now if the claim is that the minute they are filed, they ought to become available for copying by anyone, including by commercial services, that's a perfectly defensible claim. It's just that there's no existing copyright law doctrine that clearly embodies such a rule. (There is a statutory provision that says works of the federal government are free of copyright, which includes federal court opinions; and there is longstanding caselaw that says state court opinions are free of copyright. But there is no such doctrine as to briefs filed in court.)
One would therefore have to argue, as I mentioned above, that the fair use doctrine should be interpreted as basically embodying such a rule. It's just that the doctrine is vague enough that we can't be clear that courts will indeed interpret it this way.
UPDATE: Thanks to Erika Wayne for the pointer to the letter.