There's a difference between bombs and Hysteria. Bombs do damage when they leave the launcher essentially, so the projectile impacts don't matter. Hysteria does damage over time, and it simply can't end early. Even if the colony is destroyed, it still ticks away until it expires. As Kitkun points out, this happens with other ships too. The Marza for example - if it destroys the colony from the first volley of Raze Planet, it will still keep firing additional volleys. If you colonize before the ability is done, it will destroy the colony. 
I proposed a fix a long time ago, basically going back to how colonizing was in beta with a delay. The idea being you make the colony creation be delayed long enough for all of the abilities to finish ticking. I think the beta delay was 10 seconds or so, between hitting the ability and the colony being created.
But that never made it in 