its simple actually.
when linux started, the only way that it can achieve certain levels of standards, it would have to be distrobuted under the GPL. this way linux gets the amount of exposure, with a wide community readily improving it on the way. This creates an ever expanding operating system that would take much much longer with unthought of resources if not under the GPL..
I read this somewhere about one guy that spent days compiling a driver for a network card under linux, and offered it for no charge to the community... people called him crazy... but he said "I just downloaded a whole open source operating source for free and you are calling me crazy..."