ECHELON shows us that the US (and the 4 other countries involved) are happy to sidestep the law and spy on their citizens.
ECHELON did it by having 5 nations involved. If the US wanted information on a US citizen they'd pass that name onto the 4 other countries who would do the spying for them.
ECHELON was also used for industrial espionage, providing lots of information to US aerospace.