This could be the case but it's still a poor excuse not to improve. Their entire service is based on mining blocks and the transaction/rewards to be as profitable as possible for their miners. Transactions are the biggest reason for a miner to choose a PPLNS pool, so not doing everything possible to avoid empty blocks is just letting down your clients. On the flip side of that Miners exist to process these transactions, so if there is even 1 in the mempool it should damn well be in the block. I haven't been on slush for quite some time, but I think the problem has been they are big enough they don't care. There have been a lot of complaints about things like the merged mining rewards, and nothing has come of it. Unfortunately a lot of people just get complacent and don't reevaluate often enough, and if they are still make mad bank on the fees for the blocks mined without facing any real consequence where's there incentive to improve.