Both data from foreign citizens and from your own citizens are useful, but usefulness shouldn't be the only consideration.
If you have lots of private information about someone that means you have power over that person. For starters it makes blackmail a whole lot easier. In combination with data about other people it helps you determine who you have to remove from a group to make that group collapse.
Now if somebody has this sort of power about some foreigners, that's not really a big problem for you country. But if somebody has that power about the entire population of your own country, that's a bit worse. That somebody could disable any form of democracy by silencing citizen protests before they even start while covertly controlling key politicians.
If communism has thought us anything it's that you don't want anybody to have intimate knowledge over large parts of your population, it doesn't tend to end well. Oversight is a nice idea but there was already supposed to be all kinds of oversight which apparently failed. I don't really trust oversight in things this important.