It's obviously because of the contract/money. If he was simply trying out and happened to dress for a couple of games, he would've probably only lost a year or maybe no time at all. From the NCAA's prospective, I can understand why they would view that as two different scenarios.
What bugs me is the inconsistent application of punishment to the guys who don't sign anything. Rob Nolan dressed for the Lethbridge Hurricanes for a couple games and didn't lose any eligibility. Desmet played for the Calgary Hitmen for 7 games, and lost a year. Daniel Holmberg played a couple games in the SEL and lost no eligbility. A couple summers back, Steven Seigo got to practice with an Austrian League team that his dad worked for, with no consequences. I guess the difference is that Desmet saw ice time, while Nolan and Holmberg were stapled to the bench, and Seigo never dressed.
I just wish they would apply the rules consistently, but this is the NCAA we're talking about.