Countries that are friends today can easily be adversaries tomorrow and the US and France haven't had the closest of relationships.
Thank you for pointing this out. Most people don't realize it.
France dropped out as a full member of NATO in 1966, ordering most non-French forces out, and didn't rejoin it until 2009. The original purpose was so that France could potentially come to a separate peace with the Warsaw Pact nations if it didn't agree with the reasons for a war with them. This didn't last long, as a secret agreement for France to rapidly re-integrate into NATO forces was signed soon after.
However, it caused a great deal of mistrust in other countries, and France has maintained interests in other countries around the world, not always for the betterment of those outside of France. It's strongly suspected of being behind the Israeli nuclear arms program, something that was originally opposed by the United States because it risked rising tensions in the Middle East where the Soviets had a strong presence in countries such as Egypt, Iraq, and Syria. France has also been caught conducting industrial espionage in numerous countries including the United States, and there's little reason to think that they've stopped there. The country has an international independence streak much longer than that of the US, and it's not likely to change anytime soon.
Nations spy on other nations, because almost every one has been assured of something and then been stabbed in the back. Reagan told Thatcher that the US had no intention of invading Grenada even as forces were moving in, and the attack began just a few hours later. I'm sure that as much as Thatcher may have forgiven Reagan later, plans were put in place to try to catch this kind of thing at an earlier stage, whether through satellites, photography from chartered aircraft, and 'tourists' who happen to be near key locations, or through traditional espionage techniques of stealing or turning people.
It's an understood game. Everyone does it, everyone tries to stop it. They get better at both sides, and the dance continues.