Not to say that filler in anime and toku is totally justified but do consider that American shows don't always have this big overarching plot throughout the entire show's run and that one American show season is not equivalent to one Toku season.

Most American seasons don't even make it past 30 episodes. Hell, Buffy the Vampire Slayer had less than 3 Toku seasons worth of episodes in its ENTIRE run from Season One to Seven.

And because these shows are made for, more or less, kids, they have to have plots short enough to be complete by the time the next show rolls around, but long enough to fill about a year. It's very strict timing.