Paying for resolution, not permission. When you pay for high resolution public domain video, you're paying for access to a better quality copy, not copyright permission. Museums employ a similar strategy when they prohibit photos of public domain paintings but license high resolutions images. An additional problem with paying for these processed PD works is that sometimes you must enter into a license agreement to use them, further limiting your right to the material. (We discussed such licenses in this entry.) Generally, you're better off avoiding such licenses.
P.S. The public domain video we "grabbed" for this entry shows the mighty Perceptron - a device with an apparent gender-blending blindspot.