I thought this was a pretty standard type of disclaimer for .gov type systems. If somebody hacks your machine while it's connected to it they can be charged with a more serious crime. It doesn't make much sense when you're talking about a web browser, but there are other areas.
Like, say, Turbo Tax's system that sends in information to the IRS when you e-file. It's not owned by the feds, but it's considered part of their network while connected.