Federal law allows foods with fewer than 5 calories per serving to be labeled as containing 0 calories. There is tons of information available online. Some of the regulations are really interesting, especially for locally produced/distributed foods. Check out Electronic Code of Federal Regulations:
(Federal regulations for food and drug labels). Also, Guidance, Compliance & Regulatory Information
(US FDA compliance).
At risk of sounding too relaxed, I'd suggest not worrying about how many calories something like unsweetened tea contains. Your concern about "racking up" 7 calories reminds me of the disordered eating habits I developed by over-monitoring my calorie intake several years ago. Check out wannabefitgirl's thread (I'll try to link it here exactly what is so healthy about my lifestyle?
) for other perspectives on this subject.
Also, when it comes to foods that come pre-labeled (particularly for foods with more calories/nutrition info than tea), I'd recommend using the nutrition information off the box, not FitDay's information. To do this, you must create a custom food and copy the information from the box into FitDay.