Every time Kate is in public, ANYWHERE, she is representing the royal family and if she makes any faux pas, it will immediately be front page news......so to that extent, they were in a way making an official appearance.
However, I think you are being way too judgemental.....2 young women, sisters, were enjoying a tennis match but also sitting in the royal box and behaving with proper decorum. In my opinion, other celebrities could take a page out of their book!
When photos appear of you grocery shopping in Wales, going to a local boutique, or in Pippa's case, going to work, you are definitely being stalked! It's not like these are publicly announced photo ops - the papparizi have to be hanging around your home or business and following you to get these shots. I personally would find it annoying and unnerving and I admire the restraint and good humor these women show. ESPECIALLY since everyone knows what happened to Diana in Paris!