The question of A2A (access to archives) is connected to this, as well as the general question of open-source historical books. It is a perennial one in the age of the digital library. For a lot of this material, there's a charge because it costs money to conserve and/or digitize and distribute it; a lot of library digital collections for research are like this, even though what's been digitized is in the public domain (think ECCO and NCCO, Chadwyck-Healey's Parliamentary Papers Online, and so forth). So it's not so much a matter of protecting intellectual property of the original maker of the film, but covering the costs of operation for organizations that seek to make it available (with a margin, of course, if it's a private enterprise). The audience is small enough for a lot of it that the price individually is fairly high, and thus it is accessible primarily to organizations like universities. [edit: I think our subscription to the Parliamentary Papers costs about $30k per year, for example.]

I'm sure one of our librarians or museum professionals can chime in here with some good professional studies that talk about the issue of copyright surrounding digitization in some detail, or personal experience with it. (Paging /u/caffarelli ?)

I'll address some of the legal aspects of the question.

Copyright expiration is a complicated topic. The short answer to your question is that some WWI footage might very well still be under copyright in some countries, while if it is not, then it can still be legally sold.

The longer answer:

I assume you are asking primarily about the situation in the U.K.

The term of copyright for a film in the U.K. is life of the author + 70 years, if it is known who made the film [1]. Here, "author" means principal director, screenplay author, dialog writer, or composer of music specifically for the film, whoever dies last. So if the principal director of a film was alive in 1945, then the film can still be under copyright in the U.K.

If a film is from an unknown source, then the term of copyright in the U.K. is 70 years from making/release[1]. So if some WWI footage is found, and no one knows who made it, then the copyright has expired in the U.K.

In the U.S., a life+70 rule, as above, holds for unreleased films. For films released by a corporation, everything before 1923 is out of copyright. So, in the U.S., again, some WWI footage is still under copyright, and some is not.

Now, suppose that the copyright has expired on a film. Then the film is in the public domain, and anyone can do just about anything they want with it. This includes selling it. If I have a copy of a public-domain film, then I can demand any price I want to give a copy of it to you. That the film lies in the public domain does not give me any obligation to give it away; indeed it essentially eliminates all obligations with respect to the film.

If you buy a copy of a public-domain film from me, then you can give it away, sell it, etc., as you wish -- almost.

The "almost" is because of a loophole that a number of businesses have exploited. A business can do whatever they want with a film that is out of copyright, including making new things out of it. One claim that has been made is that a collection of public domain works is in fact a new work, and thus copyright can be asserted over the collection as a whole. However, such a copyright claim can be difficult to enforce; an easy defense against a lawsuit is to say that the copy of the work in question did not come from the collection in question. (Disclaimer: I am talking primarily about the U.S. here. I do not know how successful the idea of a collection copyright has been in the U.K.)

[1] U.K. Copyright, Designs and Patents Act 1988, section 13B.