If I had to guess, it would be because that's exactly the environment and process that consumers will use, so it would capture variables such as "what if the probiotic was out-competed by a bacteria living on your mug?"
replyAnd then I would assume the sterile transfer was to preserve this state of the tea as-made without changing it on the way to the lab.
So bring mugs to the lab? Make the tea in a kitchen at the lab?
replyProbably frowned on to bring in food stuff and make it in a bio lab.
replyThis makes sense. Even if you're not going to drink the tea in the office, even the appearance of making food in a lab space might well be enough hassle that it's easier to just bring sampling equipment home.
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