The problem is your sources, your reading from people who frequently like to try and avoid paying taxes. They frequently bring up and tell you half truths.
"gains or profits and income derived from any source whatever"
The government has the power to tax all gains what-so-ever, and this was decided by the Supreme court.
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=348&invol=426