If anything, a citizen is more capable of carrying out a terrorist attack, or just doing any action some foreign power wants to perform in that country.
The exception for domestic citizens seems to be just a concession for the masses and their representatives, not a pragmatic choice.
Edit: a pragmatic reasoning could be compartmentalization — keeping citizens under the watch of a separate entity (e.g. FBI) but it doesn't explain why domestic mass surveillance should be ruled out.