I'm not going to suggest that saving money is or should be a big factor here, but they are free to move Enroth back and forth to Portland as much as they want this year and next. If he's not going to play on any given night, he's better served getting the time in the AHL. At the same time, they wind up paying him less while he's down there. It sucks for Enroth, but it's a part of every you NHL player's contract and he agreed to it. At the same time, they also build a little bit of a cushion in terms of the salary cap just in case they do want to make a play for someone at the deadline.
As for the injury thing, planning for injuries is not a luxury that most teams can have. At some point you have to take on that low risk and ride with what you have, so I don't really see there being much of an issue with keeping Enroth on the shuttle to the AHL. Once we hit the last couple games of the season? That might be a different case (but even then, there are restrictions on how many total callups they're allowed from Portland).