Yep bothers me too.
But in GW2 players could achieve the same thing, correct? They knew they needed to eject from the mech before it died, so that's what they did. The net effect was the same: the mech exploded and the imp escaped.
As a plant my policy is to keep far enough from mechs that I won't get caught by the explosion, then immediately pounce on the imp. Same with Deadbeard's barrel blast. Both are risky manoeuvres similar to chomper's Burrow ability, in that they are usually suicide when performed in a crowd of plants.
Edit: OK so my burrow analogy is a little wonky, since the chomper is essentially stunned after chomping, whereas imp and Deadbeard can hightail it out of danger. And mech and barrel blast have the potential to neutralise retaliation by taking out multiple plants, unlike burrow chomp which only removes one zombie from the battlefield.