It's PEIX that can't sell any more shares (dilute) for 90 days, not the underwriters who bought up the offering. That's to protect Lazard Capital Markets, Cowen and Company and Craig-Hallum Capital Group, who are free to manipulate the share price for 90 days with no fear of subsequent dilution by PEIX, as I suspect they already have.
The underwriters were free to sell the shares the day the offering closed, or decide to hold until they felt it's most advantageous to sell them, as they see fit. Let's also not forget that they only paid $15.04 for those shares, not $16.00