And we're supposed to believe that the family would give up the chance to get a sketch from one of the country's 'best' artists because of Klaas' association with Fox? What did that have to do with anything? Get the sketch out, find the perp and deal with Fox/Klaas later.
You answered your own question. The Smart family has relied totally on the SLCPD/FBI and if the police didn't want it published and feared Boylan/Klaas would leak it, not having Boylan do it was acceptable to the Smarts. For whatever reason, the police do not want a sketch published - that seems even more obvious now that Boylan has done a sketch and it hasn't been released - probably because they don't want it to be used later as a "get out of jail free" card.