has to do with the fact that baseball has been around longer and the fact that back in the day- like the early 1900's, a lot of MLB teams would offer college guys money because things weren't as structured with the NCAA like they are now.
For example, Willie Mitchell who pitched at MSU and was the first Mississippian to play in MLB, was I'm sure offered money by teams when he was pitching for us. I'm sure it was the same for many of the other great college players of the day like Eddie Collins, Lou Gehrig, and Christy Mathewson. MLB would also do the same thing with Semi-pro players back in the same period. It was just how MLB did things, and I think that precedent kind of continues today, even though it is now more refromed in the form of the MLB draft and other free agent baseball players- seniors that don't get drafted- can not be offered money until after their eligibility expires, although teams are allowed to talk to them.
Also, the college players are not allowed to sign with agents either, BUT they get around it by getting guys like Scott Boras to be their "advisor". What happens from what I understand is that you basically hook up with a guy like Boras, but you don't pay him any money while he is your "advisor", but later on once you get paid, then he becomes your agent and then he gets paid. For example- Mark Prior I believe did this, and I also believe that Bryce Harper is doing the same thing.
When it comes to basketball compared to baseball, is it a double standard? Absolutely.