The cost of keeping a player is a pick in the round he is taken. Some managers may decide their first round pick is not worth keeping but would like to get something back vs not keeping him for nothing.
If they are out of the playoffs and put a top player up for trade and the best offer is a young lower pick player it is not an even trade. However, it is fair to both teams making the deal, similar to the NFL trading an expiring contract for a 4th round pick.
Would you see this as a fair and good trade or one sided which should not be allowed?