FWIW, I don't find the tunes to "Blowin' In The Wind" and "No More Auction Block" very similar except for the first half of the first two lines of each verse, and even there the tunes are not really the same.

"Riders In The Sky" does have a melody pretty much an identical to "Johnny Comes Marching Home" - much to my surprise, because the meter/syncopation is so different - but that's only true of the first two lines of the verse.   The second half of the verse is quite different in the two songs.

Which brings me to the question of how similar two tunes have to be before the later work is considered to be "stolen" or "copied," as opposed to merely being "derivative." Lots of music, as well as works in other art forms, is derivative of earlier works.   It's kind of hard for a melody not to have segments that are similar to or even identical to others.
The first 4 bars of "Joy To The World" (not the Hoyt Axton one) are just a backwards scale; it's only the timing that makes that not obvious. But does that mean any piece that incorporates a straightforward octave sequence is copying Lowell Mason?

And if two songs use the same chord pattern and have similar phrasing, I don't think that necessarily makes the tunes "the same." E.g., it's often said that Woody Guthrie "stole" (or borrowed) Leadbelly's tune to "Goodnight, Irene" for "Roll On, Columbia," but I don't hear the two tunes as the same, even though they can be (but aren't always) played with the same chord patterns) and they share some melodic phrases.   

I say if you hear two songs played instrumentally only, and you can tell which is which, then the tunes aren't the same.   "Great Speckled Bird" really IS the same tune as "I Am Dreaming Tonight Of My Blue Eyes," which is the same tune as "The Wild Side Of Life" and the reply "It Wasn't God Who Made Honky-Tonk Angels."    "Love Me Tender" really is the same tune as "Aura Lee." But many of the other "examples" I hear are really better described as "derivative" rather than "copied."