[R] by() subset by factor gives unexpected results

[Myles English]

I am having trouble understanding how the 'by' function works.  Using
this bit of code:

i <- data.frame(x=c(1,2,3), y=c(0,0,0), B=c("red","blue","blue"))
j <- data.frame(x=c(1,2,3), y=c(1,1,1), B=c('red','blue','green'))

plot(0, 0, type="n", xlim=c(0,4), ylim=c(0,1))
by(i, i$B, function(s){ points(s$x, s$y, col=s$B) })
by(j, j$B, function(s){ points(s$x, s$y, col=s$B) })

I would have expected the point at (1,1) to be coloured red.  When
plotted, this row is indeed red:

> i[1,]
  x y   B
1 1 0 red

however, this next point is green on the plot even though I would like
it to be red:

> j[1,]
  x y   B
1 1 1 red

How can I achieve that?

Myles