AFAICT the argument seems to be that regardless of flags (because they are to easily painted) the tanker in question is being used by Russia to evade the sanctions since 2022. The fact the US appeared to need UK help (probably to read the map) is interesting for a country so determined it can do without allies.
Russia will bellyache about the operation, because the only effective options they have are to scare the US allies or launch a nuke. (If they can).
That's my detailed analysis. However TL;DR is fuck you Putin.