Why would I pay for Linux or software for Linux? I thought it was free!!!
Linux and software that runs on it is generally covered under the GNU GPL license
www.gnu.org/copyleft.html
Software may be sold for a price as long as the source code for that application is made available for free to the community.
Think free as in speech and not free as in beer.
"Free Linux" is an intangible idea rather then a tangible thing you can touch, feel or smell, you have the right to look at and modify the source code as you wish as long as your modifications are released to the community at no cost. However the software itself may be sold for profit.