Not only that. I fully realize Koivunen and Broz might not amount to much or anything at all. But they're going to give trash like Bemstrom, Huntington and Imama 2nd line minutes over them. Yeah yeah it's just practice. But you should want your actual prospects getting all the ice time they can get.
That's fairly standard for how the AHL operates. AHL teams aren't purely just trying to give young player the most minutes possible, they're also trying to build a good team.
There are also AHL veteran lineup limits that basically guarantee that Koivunen and Broz will be playing a ton. This shouldn't matter because Puustinen should be in the NHL, but I wonder if that is a small factor in the decision of keeping Puljujarvi up over Puustinen. Puljuajrvi counts as a veteran by AHL standards (356 NHL games, limit is 310 AHL+NHL games) but Puustinen doesn't (223 NHL+AHL games), which does have implications for making WBS a better team. Basically, Puustinen being in the AHL allows them to have another veteran playing regular minutes, while Puljujarvi would take one of the 5 veteran spots.
Edit: actually this is incorrect, some of Puustinen's Liiga games do count. His 21 year old season definitely does but I'm unsure if his 20 year old season counts. He's at either "development player between 260-310 games" or "veteran" depending on whether that 20 year old season counts.