Poor sportsmanship? No, they were just engaging fully in the Cubs' traditions.

If you were unaware, the blue flag with the white L on it is as much a part of Cubs' lore as the white flag with the blue W. The tradition actually dates back to the very beginning of baseball at Wrigley Field - before even radios existed. At that time, the only way to find out whether the Cubs had won was either to hear it by word of mouth from those who had been there, or to read it in the next morning's newspaper. So they decided to do something to let people know the results of the game. If the Cubs won, they decided, they would fly a white flag with the blue W; if they lost, they would fly a blue flag with the white L. Of course, with the advent of radio and TV broadcasts, the flags fell out of use at the stadium, and when the white W flags came back into favor, the blue "L" flag was pretty much forgotten.

So the Brewers, being the traditionalists that they are, decided to engage fully with that tradition.

And also, you're kidding yourself if you honestly don't think the Cubs would have paraded a white W flag all around AmFam if they had won that game... Hell, go to any Brewers-Cubs game there - there are numerous Cubs fans in the stands holding up their white W's. So no, not poor sportsmanship. Just that the Brewers are a very giving team - they always believe in giving back to the community, so they were simply "giving back" what the Cubs' fans had given them every time they won a game in Milwaukee...