The NCAA would need to make some changes to what constitutes amateur status for hockey in order to do this. That's the really big issue re: the kids choosing CHL over NCAA.
Worth noting that there is some talk of Hockey Canada, and therefore the CHL, working towards changing the drafting rules for the NHL in concert with the league itself. Some of the proposed changes would include altering the first round to being the only in which teams can draft 18-year-olds, or the possibility of removing their eligibility at 18 altogether.
There is some talk about the legality of drafting 18-year-olds, a lot of which I am foggy on, but have been doing a bit of looking into. I think it has a bit to do with the money paid back to the CHL and the teams for the players, but I can't be sure.
Another big issue is the blocking of "signing" money, something that is said to happen often for kids wanting to go play in the CHL. Rumours are abound that in the past teams like the London Knights have paid in the six-figures to have certain players come to the OHL, WHL, or QMJHL.
The whole NCAA v. CHL argument is so deep and there are so many problem areas that it's hard to say where you could even begin to start if you wanted to stop the CHL from taking the kids from the college ranks.