It's just that when it comes to first languages, we take full immersion for granted. Our native language, by definition, is the one in which we are born, and immersed within our homes and communities, throughout those crucial first years of language development.
Immersed, that is, so long as we tune into the people around us, share attention with them, and appreciate them as intentional agents that communicate--with one another as well as directly with us.
A child on the autistic spectrum who rarely attends to what other people are attending to or communicating is like a visitor to a foreign country who takes out her powerful earplugs only occasionally, for fleeting moments that encompass fragments of disconnected discourse. Such a child might eventually acquire a basic vocabulary, and even put words together that express intelligent thoughts, but is likely to remain mired in an ungrammatical pidgin unless, before those critical years are over, his social attentiveness increases substantially.
Fully capable of mastering grammar, but immune to the full immersion that could take him there, this child desperately needs an alternate route.
For him or her--this systematic, rule-seeking child with autism--what better route to grammar--the systematic, rule-based structure that underlies every language....
...than that of systematic, rule-based grammar instruction?