A belated response to Menno's points.  The question was about the 
claim about people that say they trade for the public good. These 
people demand preferences and rewards that generally benefit only 
them. Smith's whole book is a polemic against special merchant 
interests claiming preferences/protections & subsidies/monopolies in 
name of public good. Smith does not deny that sometimes trading for 
public good can produce good outcomes for society, but he thinks that 
on balance society benefits far more from people who pursue their own 
interests/trades.

I think it is a bit misleading to speak in terms of 
optimal/sub-optimal outcomes. If one were to use anachronistic 
language, I would say that Smith's point is a public choice 
(rent-seeking) one here

Sincerely,

Eric Schliesser