This is just about Butero and William's visit; I'm not up to speed about what's going on with Harry or Andrew.
Butero is a cool new(ish) place in Warsaw, somewhat under the radar (until now - I'm sure they're happy for the publicity from PW, although they may struggle a little with an upswing as it's a small venue with local seasonal food prepared from scratch). The name, "butero", is a clue to their focus and ethos - it's the Esperanto word for butter. Their cuisine - which tries to strike a balance among vegetarian, carnivore/omnivore, and vegan (that'd be the plant-based butters) is butter-themed, and the name is a nod to Ludwig Zamenhof, a Pole who championed Esperanto (and was nominated for the Nobel Prize for doing so) as a way for people to communicate in Eastern Poland in the mid-19th century when there were equal and minor claims for Polish, Russian, German, Yiddish, and Lithuanian languages, among others.
I'd consider that all of these things might've appealed to William (perhaps suggested by whomever on his staff scout and book for him): good food ethically and locally-sourced, Polish AND pan-European (chimes with a representative of the British State visiting and supporting Poland, and with NATO - I don't say EU, but some would), multicultural, and LGBT+ friendly. I can't much fault him/them for any of these, under the circumstances of his visit or in general.
Also, for anyone confused by the menu: the fries aren't served with actual amber (what a deal at the price if they were!) but with golden-brown mustard. And for any animal rights activists, the "green hooves" are spinach and potato dumplings.