I guess GO - as any other aspect of life - must be entwined with complicated politeness restrictions in Japan. But I don't think everything applies to western GO players too. Esp. if you play on the internet.
E.g: I like to play GO, so I play a game. I practically won the game but my opponent continues. I'm happy because I'm now playing an easy game and I like to play GO, remember? The thought that continuing to play would ever be considered rude is absurd to me.
Of course when they used to play a game through days and weeks that consideration would have more merit. But nowadays (unless time limits are so set that the opponent takes very long to play each move) it's not a big problem IMO.
Sometimes despite I know I'll lose I'm still curious about the final outcome.
Sometimes I want to take my chances and fight an uphill battle and see if I can win.
So you're a sandbagger? Now I'll take this opportunity to practice against a stronger player. Yes, endgame too

.
The only time resigning should be mandatory is with the "absolute" time setting IMO. But I never play that because I think that setting is flawed.
What I don't like is when they resign after scoring. But that's not a big deal either (unless the ranking system takes scoring into account which I'm not aware of).
Sure it may happen that one loses an "already won" game. But that just means that the game wasn't really won in reality. It once happened to me that my opponent didn't want to accept that L in the corner was dead. So we played it out and I forgot to eliminate KO threats beforehand so it lived and I lost. But at least I learned my lesson and I will not forget that again.