Remember, these companies are just trying to protect themselves. The fines levied on copyright law offenders are extremely serious. No one wants to intentionally break the law anyhow, it's not good business or good for the photographers who make a living from their images. And while there may be some flaws in the way it works, there really is no better way to determine whether or not something has a copyright on it -- photographers are not required to mark their images in order for them to remain protected.
And, besides, if all of the work is yours, it's perfectly ok to sign a copyright release. Look at this as a compliment

! It's actually one of the joys of the job to print things for folks who care about the photos they are taking, most of the stuff we do is pretty ho hum. If someone in my town brought me some of their digi scrapping to print, I'd be thrilled!!