I imagine it was so that everyone would know his opinion on the matter, and that it would be pointless for Solozzo to try and get around Vito's "no" by approaching any of the others individually. Something along the lines of presenting a united front. But then, Sonny ruined it all.
Agreed and he wanted the entire family and other families to know that the Corleone's do not deal in drugs as passed down from the Godfather's own mouth.
