It's not about buying the souvenior program. This is why it matters:
There are two major protections gained by the designers:
1) Not allowing amateur video or still photography will aid in the protection of copyright, which will reduce the risk that the designers' work will not be duplicated by future productions. This may seem "silly" to some because their work is often findable in souvenior programs, press photos, and b-roll footage.
2) Not allowing amateur video or still photography means that the majority of video or still photgraphy will be circulated from approved press captures. These are professionally taken and usually showcase the production in the best way possible, considering everything from the lighting to the choreographic poses to the actor's faces. This reduces the risk that the designers' work will be shown in poor reception. (For example: Joe takes a video using his cell phone of a moment of a Broadway show, and shows it to Jill. Jill comments that the costumes look ghostly and ugly. Jill formed this opinion of the costume designer's work based off of a pixelated, washed-out video shown on a tiny cell phone screen. Had she seen professional b-roll footage with appropriate lighting, her opinion of the costume design may have been positive instead of negative.)
All of this may seem "silly" or "foolish" but nonetheless, it is a protection granted to the creative team and authors, and even if they didn't really care, their lawyers would advise them to take the "safe than sorry" route.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle
