As far as aspell-nl is concerned, we just have to assume that the word list might be covered by copyright in at least some countries, so we need a proper licence for it.
However, since a general discussion of word list copyright seems to have ensued, what I don't understand is what happens if you make a dictionary or database and in it mark the words that appeared in somebody else's word list. On the one hand, you are merely recording a load of facts in your own database (word W appeared in word list L, etc). On the other hand, anyone can generate the word list from the dictionary/database, so you are in effect replicating the word list. So, is it a copyright violation? In practice, you could probably take the union of several non-free word lists, intersect that with the union of some non-free text corpora, then randomly delete a few words from the result and maybe add a few words if you can think of any to add, and declare the final list to be the result of your own original research. Who can sue you? Edmund