It is because congress and law makers don't make the US federal agencies accountable for their actions. A law does not exist if there is no punishment for violation of said law.
It's not actually illegal for the CIA to spy on foreign embassies.
The US was caught bugging the UN, and we tapped Angela Merkel's phone. The intelligence agencies are, more or less, doing exactly what they were set up to do.
And the other guys are bugging us in return (see: mysterious cell-site simulators popping up in DC, etc).
The CIA runs a drone warfare program, bugging one embassy is not going to turn their stomachs.
