What your friend ran into was more of a "better safe than sorry" reaction from the website as opposed to a certain violation of law. Generally speaking, while the design of a building can certainly be copyrighted (post-1990) and a distinctive building can be used as a trademark (ex., the castle at the start of Disney films) case law doesn't offer rights holders blanket protection when it comes to photographic use of such buildings.
The most recent case regarding this was
the Rock-n-Roll Hall of Fame in Cleveland in 1996. The HoF tried to sue a photographer for selling postcards and posters of the distinctive landmark. The HoF claimed trademark infringement because they do use the building shape as one of their trademarked logos. A judge in a lower court sided with the HoF, but a majority on the Court of Appeals sided with the photographer. The majority of judges pointed to the standard that a trademark's use must be consistent and since the HoF itself had sold lots of posters, t-shirts, and other trinkets with a variety of different depictions and views that they had undercut their claim that any use of the building itself was a "trademark". In essence, they merely felt the photo was only of an well-known publicly accessible building. They even found his inclusion of the trademarked "Rock 'n Roll Hall of Fame" on the poster "fair use" since it merely described what was depicted in the photo.
Does this mean that Cindy's Castle is "fair game" for any for-profit usage? No, but it also doesn't mean that all uses are verboten either given that Disney's use of the structure is akin to the RnRHoF's use of their building. In matters of copyright and trademark, it all boils down to a judge/jury weighing a variety of factors in court. But often times, place like your friend's website would rather avoid any legal entanglements and costs and just say "No".
