I like to think of the various "Planar" races as being essentially bereft of true free will. They may act in ways that appear to reflect "will", but they are severely constrained by the "purity" of their existence.
When it comes to Slaad, they're more of a force of nature. "Excess" law (metaphysical sense) tends to "attract" them "naturally". Likewise, anybody who intentionally deals with them has to be very careful about building up too much "chaos charge" locally, because it can "arc" to Nirvana with unfortunate results.