I saw it as Omi didn't confront Krampus and learn the true meaning of Christmas (giving and sacrifice). So she had to live with her whole town dead.
Max understood the meaning of Christmas and confronted Krampus, so was given a second chance. You can look at it as the snow globe as Krampus' way of keeping an eye on them.
Krampus analysis video

Because of bad writting. The ending just doesn't make sense, plain and simple.
I really felt quite distressed at not receiving an invitation - Maleficent

Nobody was put into a snow globe in Krampus' lair. Those are his windows into the souls he is watching.

Omni got her wish. She would have no idea her parents were tucked away in krampus's lair via a snow globe.
The boy was different. He tried to take back his wish and in exchange he got captured also. Otherwise he would have thought his family was killed as well.

You sort of answered your own question. First of all, the family was not capture and put IN the snowglobe. They would have known, but they weren't. If they really were, we would have been shown something to the effect that they 'knew' they were IN something we weren't. If Krampus really DID put bad people IN snow globes, then almost implied by your question would pose the question: Why didn't Krampus give back Omi's parents INSIDE a snow globe? Because that's not what he does. He literally simply took them and recycled their souls to create more demonic toys. (more than likely) But simply did what Omi wanted (at the time) she lost her faith in the Christmas spirit and wanted her parents gone so he took them.
In Max's case, he did something different. He cursed the ENTIRE FAMILY, and Krampus almost gave him that except, Max did something Omi did not do, sacrifice. And as the rest of the family was being taken, they ALL were really sacrificing themselves to some extent for each other, but it meant nothing because Max accidentally cursed them ALL (except him). That's why he got the bell. At that point everything mirrored Omi's story perfectly. Except, this really isn't what Max wanted. He gave the bell back, he wanted his family back. Yeah, he laughed at Max, but then at last minute we get a close up of Krampus' face after or as he throws Max down the whole. The shadow demon 'almost' seemed sorrowful. As if he knew what it meant. It meant Max would get his life back. Everything had to be made back like it was. Even Max sort of realized that, and asked for it! See Omi's story was ONLY based on her
own
experience, she had no idea what the idea of love and sacrifice would mean to the Krampus had she known, she might have gotten HER parents back. But she was selfish and vengeful, Krampus welcomed it, took her parents, winked at her for her wish, and left back out in the snow and darkness. Which means, he's WATCHING Max's family through the snow globes. They were not captured. And this also explains why Omi was not IN a snow globe. Because she was spared too and given the bell. Max now has his own bell too, because he (along with the rest) were all spared. And this is the mentality we have to look at the movie at this angle when we have no background info from the other media based in the same Universe. They were spared and are being watched. Yeah, Omi's story is foreboding it's supposed to be, but she didn't know everything She knew OF Krampus, and what happened to HER as result of HER actions. Wish she had said Froliche Weinachten at the end oh well.
3rd generation American from a long line of Gottscheers it was Drandul, dude!