They are not alone in using the term. I found about 500 legitimate hits on Google. As I look through the listings, I'm not sure I can detect a definitive pattern. Here are some examples:
- Pre-assorted tattoo ink. The preassorted variety is not random; they explicitly list the colors they include.
- Pre-assorted cupcakes. Same: a preselected variety.
- Pre-assorted pound of taffy. There might be some randomness to the assortment, but if so, it's constrained: "We pick only the most popular flavors."
- Pre Assorted Nylon headbands x 12. Perhaps this gives us a clue: "No duplicates. You do not get to choose colors." It's an assortment, but we're doing the selecting for you.
- Jacqueline du Pre Pre - Assorted Concerts This is a YouTube playlist.
I actually have access to a kind of subject-matter expert; my daughter works at a store that sells chocolates. I asked her whether they sell "preassorted" collections. No, she said; they use the terms pre-packed or just assorted. Both of which make sense to me.
So I'm still a little unclear on what preassorted conveys that preselected doesn't. Any ideas?