I think it's because his high school in Canada played a lot of games against US high schools so it's kind of like he's a US high school prospect. In this case he would not be eligible until being a year removed from high school (unless they convince the nba of this weird post grad year thing).
If the Celtics keep all of their picks I could see Maker going with the Celtics own 1st round. I wouldn't be too upset with that. I think Maker is eventually an nba player, but it may be after looking terrible for a year or two. Depends on his work ethic and mental strength. Perry Jones III is actually a good comp I think. Without injuries Perry Jones is a solid, but inconsistent nba player. The line between this and out of the league is thin and fragile though.