OK, this comes from the point of view of someone who plays music strictly to entertain herself and whose music circle is strictly amateur.
Even if I spend a lot of time researcing a song--and I have done this on several occasions--it would never occur to me to be selective about who I gave it to unless it was someone (and there are *very* few people who come to mind when I say this) who might abuse the favor. After all, who is to say that the next time I'm trying to track down a song, the person I wouldn't share my last song with isn't the one that knows something about the song I need now?
There is a family among my current musical acquaintances who is notorious for "grabbing" songs--they arrange them to the Nth degree, insist on leading everyone when we play, make everyone buy their tabulature (none of it for original compositions), and are vague about background research (actually, they've started giving credit recently, partly because I _always_ do and I shamed them into it). I *occasionally* rearrange--not just recopy--their stuff for people who are either sick of having to buy public-domain songs or don't like the arrangements for sale. I would never record or publish somebody else's original song or arrangement of a PD song without their permission.
Personally, even if I learn a song from someone, I have a lousy memory and I usually "rearrange" it completely by accident, anyway. You should hear the unrecognizable version I sing of "Old Bangum", filtered through at least two, maybe three people who couldn't remember it right.