This is not in anyway a trial with open source software. FOSS developers work on software that they're familiar with. I doubt very many of them own thin client hardware. They also don't like proprietary software and on the other end of this client is VMWare's closed-source server software.

VMWare is open sourcing this product because thin clients tend to run on exotic hardware and they want to allow manufacturers to easily port and install their client. That ensures that they'll be locked into VMWare's server product.