They sure do but I can understand the reasoning. Originally their scans were free of watermarks but it did not take long before the backgrounds appeared.
Soon after I posted my first manuals on this site those manuals appeared on a scumbag site that was selling copies. Obviously someone from that site is a member here and thinks that selling goods that were donated to all is a good business model. I no longer visit scumbags site and always suggest people use ethical sites like
WWII Era Military Blueprints & Manuals | AirCorps Library who publicly acknowledge the suppliers of all their manuals unless the donor fills in a form requesting anonymity. Yes they charge an annual or monthly fee but they do not steal their content.
That is why I always watermark everything I post with ww2aircraft on every page. I hate doing it and I know that some scum still sell them on their sites but at least they cannot remove the watermark so people know they are pirated and where they came from. Maybe we get new members from this. Maybe not. Hopefully any new members are willing to upload their less common manuals like I do. Generally I do not post Spitfire Mustang etc as every man and website and their dogs have those manuals.
My in-laws have a bigger problem in that one family member is the author of over 50 books and most of the copies available on the internet and even in bookstores are pirate copies. At least with Amazon the family still gets the income from pirate copies though it means a monthly search for what new ones are being pirated. More than one scumbag "print on demand" supplier leaves the authors name and copyright info etc on the scans they use and then put
Copyright Scumbags Incorporated in large bold print on the bottom of each page
The problem we have now is that many of those who have good collections of manuals are passing on and the first thing the kids do is dump all the manuals in a dumpster. One collection I heard of too late the !@#$%^* had just poured lots of sump oil all over the contents. I salvaged a first edition of
The Rubaiyat Of Omar Khayyam from 1859 because it had minimal damage being so small and near the top. There was a mountain of WW2 RAF Air Publications in there but none were worth keeping. I did enjoy telling them that each of those RAF pubs was worth at least $100 and the rarer ones several thousand dollars. In reality a rare book seller would have offered them more for the book of poetry than the manuals.
My collection is willed to a museum and my children and executors (and recipient) are fully aware of this.