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.