That's clearly untrue. Otherwise we wouldn't have the CIA as an organization to spy on foreigners.
It is true you don't need to be a US citizen to have protection. However, you do need to be under the jurisdiction of the US (on US soil). Otherwise, the US Constitutional rights don't apply to you.
No, the applicability of Constitutional rights outside the US has yet to be completely settled. The Supreme Court discussed that issue somewhat in Hernandez v. Mesa in the current session. I suggest listening to the oral arguments on oyez.org. It is a sad and infuriating, but also fascinating case.
The CIA doesn't feed or perform as a part of the United States Justice system (parallel construction or the occasional slip aside).
Therefore, using it as a argument for why due process is only restricted to a certain group is a bit of a non-sequitur. A case against a foreign person still has to be conducted through proper channels, either with Constitutional protections in force if the U.S. has ultimate jurisdiction, or through diplomatic channels to ensure conformance with whatever other country considers due process or actionable if the U.S. doesn't have ultimate jurisdiction. This doesn't really negate the GP's point in the way you may have been intending to.
While the government technically isn't allowed to do some of these things, the way our judicial system works, there's no lever with which these can be stopped (other than at the ballot box, and it seems no one cares enough to vote based on this). In particular, what little controversy there was surrounding the Edward Snowden revelations showed us that the Courts don't recognize any of us as having standing to pursue the question in court.
It is true you don't need to be a US citizen to have protection. However, you do need to be under the jurisdiction of the US (on US soil). Otherwise, the US Constitutional rights don't apply to you.