@mojo: Not really. People deserve to get sick IF they are dumb enough to buy food from questionable sources. That's why we have brand names, Royal Warrant, and that's why shops often put "est. in 19XX" to show that they are not a fly-by-night operations selling dubious quality goods. That's also why reputation is important as a merchant. You don't want to build a reputation for selling bad stuffs.
Of course in pre-modern era you don't have regulations, and people often cheat. And that's why they had guilds with their strict membership system, because they didn't want fly-by-night operators/swindlers coming in (see https://www.jstor.org/stable/26654178?seq=1#metadata_info_tab_contents). Pure Food and Drugs Act came into being AFTER industrialization, with rapid urbanization, making it difficult for people to find out what is in their food.
So in the case of that merchant, if those nobles knew that they got sick from drinking something they bought from the store, the first thing they should do was to either call the guild or the police/imperial guard/whatever security system in that Isekai world.
Moreover, as you mentioned, these nobles "wish to follow the trends of society" meaning that they have pride to maintain. And it is doubtful that they can maintain their pride by telling people that they got their tea not from the trendsetting store that has a Royal Warrant, but from "a middle class looking store."
@LegendX: the other merchant's store is not a café, meaning that they were buying raw tea, not a cup of tea.