It is hygiene, for it enforces cleanliness; it is economics, for it shows comfort in simplicity rather than in the complex and costly; it is moral geometry, inasmuch as it defines our sense of proportion to the universe.
I have not been to Beijing yet, so I don't know any good place to buy tea. But I definitely would stay away from tourist area and tour guide led places for tea purchasing. These places normally charges hidden fees for tour guides and a premium for tourists and non-local people. If possible, have some trustworthy locals to bring or refer you to places.
Also, make sure you taste the tea before you purchase and don`t get carried away by the beautiful fancy packaging. Look at the quality of the actual leaves. There are many cases where the packaging is very nice, but the tea is crap and vice versa.
As for brand, I would purchase Menghai or Xiaguan tea factory Pu-erhs, but I am not aware of other well known or reliable brands for other types of teas. Depending on the tea you are interested, there are different attributes to look for when selecting the tea.