The Linux kernel was never proprietary.
"Since the initial release of its source code in 1991, it has grown from a small number of C files under a license prohibiting commercial distribution to its state in 2009 of over 370 megabytes of source under the GNU General Public License."
And by the way, the GPL was first written in 1989, so you could hardly argue that in 1991 many people understood how free software should work or which are the longterm consequences of using a certain license for your hobby project. Cut Linus some slack, he released source code and eventually used the GPL, right?