SH2 for certain is not "public domain"
Ubisoft US Store SH2 for sale
Near as I know, Silent Hunter (the original) is NOT public domain either, though it is not for sale on the Ubisoft site. However, you can buy a used copy on Amazon for $50US, and the one page is still linked to the Ubisoft Amazon store - for what that's worth...
Noting the "the copy protections scheme was a huge mess" comment, I have owned the game since pre-release, a v1.0.x (whatever) DVD. I also purchased a v1.3 disk, and a v1.5 U-Boat Missions Add-On, as well as several copies of the "Gold Edition" in various formats. I never had an issue with DRM with any of them at all. You did have to keep the disk inserted in the drive for the early versions, but that went away with v1.3 in 2007. I also had SH3 with "StarFarce" (StarForce) DRM. Now, that was a weird one, but SH4 has never had issues like that. The biggest thing with SH4, and it is the same with any other DirectX v9.0c Windows XP and earlier games, is that Windows Vista and later added "protection" to the OS. If you install the game outside of the Program Files and other "protected" folders, no problemo. Never accept the default install path - but you must also install DirectX v9.0c and its codecs for those old game to function on the 'modern' computer.. The game has been "on sale" recently for as little as $3 US through Steam and Ubisoft both. It shouldn't be long and one or the other (or all of them) will price it there again...