There are tons of FOSS/OSS projects for which OSX is the main platform.
Windows isn't exactly a "proprietary" platform, the OS is closed source and proprietary is but it's the wrong definition to use to describe it as a platform especially today with PAAS/IAAS.
Windows isn't free that's true but as far as PAAS (Azure/Azure-Like) goes it's not really a critical part of the pricing, and even for IAAS it's not that important.
One could also say that RHEL, SLES and the likes are proprietary distributions of Linux, they are open source, but proprietary none the less.
Windows isn't exactly a "proprietary" platform, the OS is closed source and proprietary is but it's the wrong definition to use to describe it as a platform especially today with PAAS/IAAS.
Windows isn't free that's true but as far as PAAS (Azure/Azure-Like) goes it's not really a critical part of the pricing, and even for IAAS it's not that important.
One could also say that RHEL, SLES and the likes are proprietary distributions of Linux, they are open source, but proprietary none the less.