That is definitely not fair for the players. And it makes no sense for the league. Your saying that every UDFA, like Blackshear, who signs as a rookie to try and make a team and then gets cut, will cost a 2nd team a draft pick to sign them now. If the 2nd team wasn't willing to use a draft pick on that player during the draft, why would they do it now.
Plus if every UDFA that was picked up by 2nd team required a 7th round pick, there would be more than 100 7th round picks.