Sponges do not remove powder snow

...and why should they?

Because powder snow can be in a bucket like water. 

Snow is also made of water in real life so it makes sense. 

Lava can also be in a bucket, yet is immune to sponges, and snow and ice are made of water yet have no reaction to sponges either, so I don't see the point you're making.

See MC-126978. 

And that was corrected.
Sponges are only intended to soak up water blocks, per changelogs and that fix. This is a feature request.

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