Also there's fundamentally a difference between playing against other players and playing against the house. If the professionals on draft kings / fan duels got to play the house in a similar setup it would be even worse for the people playing for fun.
Why are people allowed to go to the movies? In New York a movie ticket is $15 and 90% of the movies are, in my opinion, absolute crap. So that's $15 with a 10% chance of getting your moneys worth and a 90% chance of losing all of it and 2 hours your time!
I think the one reason why it's different for the leagues is that daily fantasy doesn't really create as large of incentives to throw games or rig games as much from the refs or players point of view (not that it's common, but it's definitely happened in the past).