Hadoop is free software
"Hadoop, the open source data-crunching platform based on Google's proprietary infrastructure."
I think that this sentence is misleading. Hadoop is an Open Source software, that does not contain a line of proprietary code, nor implement any proprietary technology or infrastructure.
Hadoop was however inspired by the MapReduce concept, that was first drafted by Google researchers. The first implementation of the algorithm/framework was done within Google walls. It was, and still is, closed-source software, a.k.a. proprietary software.
So Google's implementation of MapReduce is proprietary. Hadoop implements something that was originally inspired from MapReduce. But Hadoop has nothing to do with Google implementation or infrastructure.