Asklemmy

the flag is flipped (for whatever reason the text is in the same position on both sides and not reversed), which is perhaps why it's difficult to read, the second word is definitely клуб in reverse, and the first word begins with ч, and has two й, so most likely the first word is чайный

so my guess would be it's чайный клуб (tea club)

i assume this is their website, and if you look at the header you can see a face with a monocle that imo looks really similar to that of a flag shown in the video

from a quick read of their website it's a movement that separated from the libertarian party of russia, and the name is a reference to the boston tea party

as to why that person was targeted in particular: don't try to rationalize or attempt to seek logic in police behaviour, they are just selectively arresting anyone they choose, people with flags or posters in particular