They have been using the "person of interest" line for a long time to avoid some of the legal rights that attach once a person is considered a suspect. I think the rules relating to how they can be questioned changes for instance.
A material witness can be held indefinitely without ever being charged with a crime for instance, and doesn't get the same level of 5th amendment protection as a suspect who has been accused of a crime.Sounds an awful lot like semantics to me. If a person of interest is being questioned it sure sounds like they are a suspect. I mean hell, can't you have a dozen suspects in a case?