The police can conduct covert surveillance through all sorts of methods not available to the NSA.
And the NSA can do all sorts of surveillance not available to the police. Big deal. You are missing the point. The precise techniques used are not important here. What is important is who is watching the watchmen. I don't give a damn THAT the NSA is listening and generally don't care much HOW. I do however care about WHO they are listening to and WHY and most importantly WHAT is preventing them from abusing their power. A secretive organization operating a secret program to to spy on unknown parties under secret directives which is overseen by a secret court that issues secret rulings is not what I would call accountable.
Quite frankly I have FAR more to fear from the NSA than I do from any terrorist. My government has a very recent history of torturing individuals, indefinite imprisonment without trial, targeted assassinations, justification of doing so again US citizens, "extraordinary rendition" and more.
The NSA is specifically targeting people overseas with this program, that has been clear
That is NOT even remotely clear. We don't know what the NSA is targeting. We know barely any meaningful details and none appear forthcoming. We also have no transparent oversight. The only thing that is clear is that we don't know very much and that is what is worrying. An unaccountable government agency is a very dangerous thing.
I remember reading an article about how in order to subvert email monitoring systems, the terrorists would open up a webmail account, write an email in the account but never send it, leaving it as a draft.
You do realize that is EXACTLY how David Petraus (the former head of the CIA) was caught cheating on his wife, right? If it is online, it can be seen. Only an idiot would think an unsent draft in a gmail account was somehow secure against anything.
If they admit they are monitoring communications like this, even at an extremely basic level and collecting the information described, then the whole system becomes worthless because the bad guys they're trying to capture will just not use the system, and then there's no point.
That is an incredibly naive view of how things work. If these bad guys truly are so dumb that they would only secure their communications if we admit we are listening to them then it should be very easy to find them. Some of them may be that stupid but any of them worth worrying about probably are not. They KNOW the NSA is listening and it's not hard to guess how they might be doing it. That does not however mean that the NSA has a right to hide that fact from the very people they are supposed to protect. NOTHING is gained by keeping the mere existence of this program secret and there MUST be some means by which the actions of the NSA are made accountable to the citizens.
This is no different than what the Allies did with Enigma and Magic
It is vastly different because the NSA is monitoring ME without probable cause. I don't give a crap if my 4th amendment rights are inconvenient for the NSA. They are there because power gets abused if left unchecked.