Because everyone (T's more so than F's obviously) uses logic, even if not very well. And plus, most F's will develop some Ti and Te in their lifetime in response to their environment. And vice versa.
Anyway back to the question, for me, I'm very analytical, so even though I'm (pretty sure) 'F' most of my decisions have really strong bases in logic. Even if occasionally I might make the decision mostly on what I feel, and then think of logic for it later. But that's how I've been raised, in my family if I was totally 'F' I would be having huge family problems.
I'm using abortion as my example, and I'm a weird INFP so don't take this as typical. I think when I first decided to be pro-abortion, I just did it because it coincided with my other values, and I thought that individual women, not men in Congress, should determine that fate of themselves and their baby. But later (a couple of years), thinking about it, my F took over and I felt bad because I began to imagine what would've happened had my mom had an abortion, because you really are denying someone a potential life (even if they don't know it yet).Why do we choose the values we choose? Is it simply because they "feel right" or is there something more complicated than that going on? How do you evaluate one value over another? If someone asked you to justify one of your values could you do it, using strictly F, not supporting it with T? What would a debate between too Fs with different values look like if they defended it using only F?
But I think the core values for me I originally decided on when I was younger through my feelings when I was more F than I am now. And now I habitually make decisions that coincide with my former values, because they're so engrained in me I can't really do otherwise, and then possibly go back later and rethink them.