because tannerite, produced by a licensed professional explosives company, is considered to be for sporting purposes. ie, it is a reactive target used to hone the precision shooting skills, whereas a dry ice "bomb", as you correctly put it, does not fit that profile.
and further, a "bomb" by it's very nature is used most often for illegal and destructive acts.
i do realize that if you're just horsin' around, that no one's gonna get hurt, but too many mailboxes, etc., have been blown up with this stuff, because mostly unthinking kids use it , and that's just how it is catagorized.
can you name even a single legitamate use for a dry ice bomb?
"it gives me wood to hear it explode" doesn't count as an answer.
edit to mention that tannerite is a fine product from your native state.