Something to keep in mind is that sometimes a pool will be paid directly to include a transaction with a small fee.
From what I've seen, that fee is not "small":
I'm not sure if they share those "sales" with the mining pool users though.
I'm pretty sure that viaBTC provides a limited number of requests per hour for very small fee transactions to be included in their next block. Services like this may result in smaller rewards for any single block but may help encourage and incentivize the use of Bitcoin overall which can result in higher fees and overall higher revenue in the long term.
My assumption is they use the free transaction acceleration to "advertise" their paid service.