I don't know of any sites that routinely watermark images that they host. I think if somewhere like imageshack or Flickr routinely did so there would be an outcry. Or perhaps more simply, people would go elsewhere.
Unfortunately I cannot comment on the Campybike site as the link didn't work for me. But as far as your quote from there, I would say they have a cheek to include a copyright warning. Something of an ambit claim at worst, misinformed at best. People put all kinds of pseudo-legal disclaimers on their sites, many of which are unenforceable or have not legal power to annul the statutory rights of the individual. In Campybike's case, perhaps it is leverage to try and induce people to pay for the service?
As for my experience, I am not a copyright lawyer, but I do have an informed and professional interest as I was once a copywriter (as in I wrote advertising copy both freelance and under contract) and now am a high school teacher in the Media/Arts realm where the limits of student (and teacher) use of internet material is a hot topic and still being contested. I am also a Masters student myself so I am wary of "fair use" provisions for educational purposes under Australian Law.
Which is part of the problem I suppose because different countries have different laws.
In the case of a bona fide museum, they are institutions that are genuinely au fait with copyright law and its limits (and limitations). This is why most museums don't digitise and make available their entire collection online. But most are looking for ways of making their collections more available with some controls. An example of this is in my state, the Education Dept has cut a deal with some of the major museums to make content available to teachers and students, mainly because all Govt school teachers' identities can be verified online and they all have the same single access system. However this scheme has been slow to get off the ground because still the content being offered has varying levels of copyright restrictions applicable; from you can download it and do whatever you like to it, through you can download and copy but not alter, to you can download but not copy, to last of all you can only temporarily cache for viewing but not download and store. There is also conflict within two of the Govt's content portals: one says teachers can upload original content and that they license the Dept to use it but the teacher retains the copyright, and the other says anything a teacher uploads becomes the property of the Dept. A bit like this site actually, which says that "all content is (C) Retrobike Ltd unless otherwise stated." (I better not want to present this rant as part of my PhD in future or I might get sued...
Finally I know it is standard practice nowadays for broadcasters to watermark everything they broadcast. I'm not exactly sure how this is negotiated between TV networks and their content providers, but I know filmmakers in particular HATE the practice (as do plasma telly owners). You think about it, a large team of professionals toil over creating technically and artistically superb images, only to have every single frame smeared in the corner. They may as well not bother checking the film gate and leave hairs in it to jiggle at the edge of the picture like an old home movie. I also love it when one network re-broadcasts a feed from another network and they have to blur the original watermark to fit their own. It's a branding battle, with content the loser.
Whether its legally right or wrong, I think it is, to use a very English term, a bit common. Would you whack a big decal of your name (or your family crest or personal 'corporate' logo) on the main tube of your best road bike, just because you own it? Some people might, just for the possibility that someone see the bike and wonder, "Did Bob Jackson make that bike for Joe Bloggs? Wow, he must be important."
That's pretty much both of two cents spent, for what they're worth.