User blog comment:Omega natsu2/Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Predictions./@comment-4829046-20140214084359/@comment-24066070-20140214093309

Like an FT mage would destroy an enemy base while their nakama are still imprisoned there.

If Cana had carded (is that even a verb?) the bomb, she would've killed or injured the prisoners had it been thrown at the cube.

And just letting it detonate in the card may still cause the guild to be destroyed as these cards aren't completely sealed off (Lucy was freed from her handcuffs by a guy trapped in Sho's card during the ToH arc)