I'll confess to only knowing the surface concepts, not the inner technical workings, so I could be wrong here. But this is my understanding.

With a future difficutly bomb in the current implentation, when a proposed hard fork from the ethereum team is approaching you have 3 options:

    1) You do nothing (you don't vote - the default) - You'll end up on the chain that eventually get's it's difficulty scaled up and will die. You will not be able to continue running this chain practially after this point.

    2) You agree with the fork (a yes vote) - You need to actively install the update.

    3) You disagree with the fork (a no vote) - You need to activly install a competing patch to remove the bomb. 

(Assuming we are talking about a widely controversial fork, not just something that you alone oppose. I'm not talking about having to create a patch yourself. I'm talking about the situation where there are two competing sides, both creating their own patches. So the difficulty involved with voting either way is exactly the same, just pick a side and installing their update.)

Having the difficulty bomb guarantees that (eventually) all users post fork must have actively made a concious choice one way or the other and upgraded. The users that do nothing are guaranteed to end up on a dead chain, and hence have not cast a vote in either direction.

I'm not sure how that can be interpreted as the do-nothings casting yes votes. Anyone who does nothing becomes no part of any future chain.