Actually with the Scirocco, I asked my German colleagues (I work for a German Engineering company), to ask the local dealer what the deal was with the hydraulic bonnet lifter.
Apparently they sent a question to VW headquarters in Wolfsburg and had a reply.
Basically it said that hydraulic bonnet lifter was removed from the Scirocco during NCAP crash tests where they found out due to how low it was from the ground and it's position and angle in the engine bay when closed, in a frontal impact, in some cases the strut penetrated the bonnet and therefore caused a serious pedestrian issue in the case of the car hitting someone head on. It was apparently a last minute change by VW in 2009 (if you believe all this).
On the Polo, I believe it is just cost cutting.