Most of the times, they are alts. Anyway, once I was asked by one of my friend to send him little ETH for covering his fees. I sent but later I thought we both have accounts in bitcointalk and what if someone just links us as alt?
Then just prove that you sent him ETH for covering fees. Its not like that the feedback is not removable or is it?
Can't speak for others but I usually remove my feedback if someone has a solid proof that makes my connection invalid.
It can be proved but most of the times if someone complains that "I just sent eth for fees to my friend", "I know him from internet and helped him", no one will care. In fact the true alts sometimes accuse that they are not alts. Therefore, in reality, it can't be proved.
One of my said alt, used in my negative feedback, isn't my alt but I have to carry this accusation. Just because if I claim, people will laugh at me. That's why I even don't ask to remove that acc, lol.
EDIT: Continuing my investigation
nice find, Oscar goes to you.