If that really bothers you, there are ways to deal with it.
Say, for example, that I am going to Epcot next week and want a FP for Soarin and nothing else. I could:
1. Just select Soarin and proceed.
2. After asking if I really want to proceed with just one choice, the system will offer 2 other FP's to me in my options, and I can select an option that has the time for Soarin that is closest to what I want (I could also change this time later if I'm trying to fit a very specific slot).
3. Once the 3 FPs are assigned, I can go back to the page with my reservations and go to "update FP+".
4. On the next screen I select cancel, and the system will allow me to select and cancel just the two I don't want, leaving just Soarin in place.
It would be nice if it was more straightforward (like allowing you to just pick one in the first place), and maybe that will come some day. If you don't want to go through these extra steps, just pick two other attractions that almost certainly won't sell out, like Captain EO or Imagination. The chances that your FPs to those are going to keep anyone else from getting what they want are pretty slim.