Not sure what exactly that buys in terms of security.
Both Slack and Amazon can steal all the data without being detected, regardless of whether the servers are encrypting it or not, since if the server can decrypt it, so can Amazon (just scan VM memory for the key from the hypervisor).
Their reason that they couldn't search through data if it were encrypted is not true. They are using AWS, and can simply use EBS encrypted volumes (http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSEncryp...). Should be an easy flag to turn on.