as long as they have access to their wallet or those bitcoin addresse's private keys the dust is there maybe they can use it in a pool of other addresses to sweep them . but they dont care about those amounts since they are earning a lot more it is not even seen.
Check my previous message quoted below and you will know whether they care or not.
Amounts can't be smaller as compared to unlimited number of customers that these places have.
They sit there like dust, but will be worth something.
But as of today, nobody is worried about those few satoshis, just think that if 10K customers on an exchange leave 0.0003 in their account and forget, the exchange will have 3 BTC saved with them for nothing.
So, we should not compare it with "dust".