The Root on Singed's W doesn't work on unslowable targets

When Singed flings an enemy into his W, it is supposed to root them. This, however, doesn't work on unslowable targets (think Master Yi or Sejuani). This means, that a Singed players potential to outplay a champion who can ignore slows in certain scenarios is drastically and unfairly reduced. My guess is, though I am not a programmer, that this happens, because the root is set to apply only when it is registered, that the target is slowed by the W after being flung into it.