IMO, the answer is probably less subtle than you're suggesting: Sollozzo was taking advantage of what he perceived as a c***k in the family's armor. Puzo suggested that Sonny was careless, not that family loyalty, honor, etc., were BS. For recent discussions about Sol and the drugs meeting, see here:

http://www.gangsterbb.net/threads/ubbthreads.php?ubb=showflat&Number=490960#Post490960

and

http://www.gangsterbb.net/threads/ubbthr...true#Post444618