Drug residue? I would think there would be a ton more items that would have drug residue than a piece of mattress. What would they be trying to prove with drug residue anyway? They don't need proof that drugs were there. I'm sure the first 10 minutes search of that house could have proven that. What they need is proof of what happened to sweet Bella.
I am sorry to say, but my best guess is that there are bodily fluids on that mattress, perhaps even blood stains, and they took it to determine whom the blood and/or bodily fluids came from.
I have seen many a mattress be confiscated in cases here on websleuths (Zara Baker immediately comes to mind). Unfortunately, they tend to be for the same reason. :-(