This only seems to make sense in retrospect, because they ended up becoming the second or third best selling act of all time. It feels like they should have been willing to give some support to Muddy Waters and Howlin' Wolf, but it was clearly completely normal and accepted not to credit their sources. Howlin' Wolf's widow sued them after he was dead, but he copyrighted Little Red Rooster and accepted royalties on other people's covers even though he didn't write it and *admitted on TV* that he learned it from somebody else, who I believe he identified by name (that last point might be wrong though).
One thing I always wonder, and this applies both to black blues musicians and British invasion groups, who decides which songs get credited and which get "plagiarized." It is not, with any consistency at all, based on how much the song is changed. And it is constantly inconsistent even within the same album.