Devil buys winter coat and scarf. http://www.bit-tech.net/news/bits/2016/11/17/microsoft-linux-foundation-membership/1
Probably by helping to ensure their server products work properly on Linux, and that Linux works properly on Azure and Hyper-V VMs. No-one's gonna buy MSSQL for linux or pay for Azure to host their linux VMs if none of it works properly or is a ballache to set up.