If they cannot establish if a US Person is party to the conversation, they should assume one is and remove the potential of violating this constitutionally guaranteed right. This could be due to a failure of the service provider (Google, Verizon) to collect this information (they don't), or the fact that the communications are anonymous or encrypted, etc. Proving a negative can be tough, but that burden is the government's to bear.
Of course there is also a moral component… I'm pretty shocked that as patriotic as we are about the sacred rights of the constitution, we somehow feel that it is morally ok to not afford these protections when dealing with foreigners. The Declaration of Independence says:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
With the rights to be enshrined more specifically later in the Constitution. How can anyone passionately agree with these words and not believe that the just treatment afforded to US citizens should somehow not be afforded to non-citizens, or citizens who choose to use a particular software program?